What is the scriptural basis for the belief that a man is not capable of being the Saviour?

I am not learned in theology, but I would like to offer an answer to this question on a broad basis. That basis is that those who recorded the Gospels and the rest of the Bible were not logic-students of A J Ayer or G Ryle: what they wrote appears to have been conceived -- and it can be understood and perhaps even ought to be understood -- not on a level of strict or mathematizable logic, but on a level of openness to the inspiration and intuitive perception of its basic essential and moral messages.

In this respect I feel specially grateful to Mike Borden for pointing out in his answer how some of the Scriptural text cited here has "much more to say about the Righteousness of God in dealing with sin than it does about the constituent natures" [of Adam or of Christ]. As far as I can see, the basic essentials of the biblical messages -- bypassing any analysis of strict logic, and shining through the words themselves, in whatever language they appear -- these basics are in the demonstration and message of the hope of redemption given to us by our Lord and Savior Jesus Christ, in the words and events reported in the Gospels. And the core of that seems to be that we have been graced by God with this message of hope emphasizing that while human nature is essentially fallible and corruptible, as portrayed in the Old Testament, the same human nature is also essentially redeemable as shown in the New. And while I don't see that this tremendous message of hope is undermined in any way by those many signs that we all remain fallible and may need redeeming many times, I do see signs that it can be undermined by an excess of logical salami-slicing of the texts of the message itself.

We may wonder and puzzle about the details of the words of the Gospel message, and I don't attack the legitimacy of logical enquiries such as those exampled in the question: it is surely good to enquire and question. But I do suggest that it is important to regard the products of such cogitations with some humility, and to offer such conclusions as they may suggest without taking them to hardened extremes that can promote conflict: especially to try and learn any relevant lessons from history. There are indeed some very hard lessons of history embedded in the matter raised here: among the sequelae of the history of trinitarian and anti-trinitarian disputes since the Council of Nicaea in AD 325, and perhaps even before, is that the actions of those who took their theological views to hardened extremes on the doctrine of the Trinity led in fact to untold division, sectarian persecution, suffering and death. Unfortunately it does not seem impossible that similar causes of division and destruction of Christian and human fraternity may act again (though may God forbid it).

Part of the heat of those age-old disputes was in the past clearly ignited by the suggestion, sometimes made an axiom -- and repeated again even in the present question and discussion -- that it is somehow impossible that a man should bring the redemption promised in the new Testament.

Whence comes this stumbling-block? Christ emphasised His humanity many times, and not only in the expression 'Son of Man' which He applied to Himself. What He did and said was clearly not impossible for a man sent by, inspired by, and the son of, God. That seems to me to answer the specific current question in the sense that Scripture does not support any conclusion that a man could not be the Saviour. Can any suggestion to the contrary, in effect contradicting one of Christ's own self-descriptions, be of any constructive character leading towards truth? Surely in any event we do not need to be distracted by debates about the precise nature of Christ? The trinitarian/anti-trinitarian point can evidently be put in many ways. These included the age-old way, as a confrontation between an envisaged nature 'one in substance with the Father' (Greek 'h*m*ousios') versus another envisaged nature 'similar in substance with the Father' (Greek 'h*m*iousios'), as well as more modern clothings of similar imagined oppositions of ideas. (It seems to me that they had some good points, those who called the core of the old dispute the 'jot or iota of difference', so as to minimize the difference between those two similar but fatally different Greek words.) But why should this tangle of seemingly opposed ideas -- either way -- overshadow the central Christian message of human fallibility and redeemability with which God has graced us, potentially overshadowing also the work we all need to do to try and take up the opportunity pointed out by the message of hope, to make something good with it?

So the main point of this answer, besides offering an explicit answer as in the previous paragraph, is to try to advocate for debates such as the present one always to be accompanied by express acknowledgement that their substances are subsidiary in character and leave unaffected the central and vital message that shows us our hopes of Christian redeemability and redemption.

John 15:4-5:

Abide in me, and I in you. As the branch cannot bear fruit of itself, unless it abide in the vine, so neither can you, unless you abide in me. I am the vine: you the branches: he that abideth in me, and I in him, the same beareth much fruit: for without me you can do nothing.

Neither in the natural order nor in the supernatural order can we do anything without God's confluence. To think otherwise is the heresy of Pelagianism.

^{See: Réginald Garrigou-Lagrange, O.P., Three Ages of the Interior Life, pt. 1 The Spiritual Organism, Article V—Actual grace and its divers forms, A—The necessity of actual grace}

I'll offer a Latter-day Saint's perspective.

This topic is addressed by Amulek, Abinadi, and King Benjamin in the Book of Mormon.

From Amulek:

9 For it is expedient that an atonement should be made; for according to the great plan of the Eternal God there must be an atonement made, or else all mankind must unavoidably perish; yea, all are hardened; yea, all are fallen and are lost, and must perish except it be through the atonement which it is expedient should be made.

10 For it is expedient that there should be a great and last sacrifice; yea, not a sacrifice of man, neither of beast, neither of any manner of fowl; for it shall not be a human sacrifice; but it must be an infinite and eternal sacrifice.

11 Now there is not any man that can sacrifice his own blood which will atone for the sins of another. Now, if a man murdereth, behold will our law, which is just, take the life of his brother? I say unto you, Nay.

12 But the law requireth the life of him who hath murdered; therefore there can be nothing which is short of an infinite atonement which will suffice for the sins of the world. (Alma 34:9-12)

Amulek teaches the infinite nature of the atonement of Christ, and that it transcended anything a limited, imperfect human could do.

From Abinadi:

And now Abinadi said unto them: I would that ye should understand that God himself shall come down among the children of men, and shall redeem his people. (Mosiah 15:1)

In this chapter Abinadi is explaining to the court of King Noah how it can be that the suffering servant of Isaiah 53 & conquering king from earlier in Isaiah can be the same Being, the very Being who gave the Law.

From King Benjamin:

7 And lo, he shall suffer temptations, and pain of body, hunger, thirst, and fatigue, even more than man can suffer, except it be unto death; for behold, blood cometh from every pore, so great shall be his anguish for the wickedness and the abominations of his people.

8 And he shall be called Jesus Christ, the Son of God, the Father of heaven and earth, the Creator of all things from the beginning; and his mother shall be called Mary.

9 And lo, he cometh unto his own, that salvation might come unto the children of men even through faith on his name; and even after all this they shall consider him a man, and say that he hath a devil, and shall scourge him, and shall crucify him. (Mosiah 3:7-9)

King Benjamin taught that the atoning sacrifice of Christ entailed more than giving His life, but that He would suffer pain to a degree no mere man ever could (see also Alma 7:11-13).

That Jesus lived as a man in a body which could die is explicitly acknowledged by King Benjamin 2 verses prior. But He was not merely a man. Jesus was born of an Eternal Father & a mortal mother, inheriting attributes of both. He had the ability to die; He also had the ability to rise again. He had the ability to suffer pain & fatigue; He also had the ability to endure & overcome all things.

--

Conclusion

Latter-day Saints believe that part of the atonement was Jesus' suffering in the garden of Gethsemane; we believe that in Gethsemane Jesus was not just contemplating with anguish the pains He knew would come a few hours later at the hands of the Romans--Jesus experienced something considerably worse. He took upon Himself all of the pain, all of the sorrow, and all of the grief all of humanity would ever feel. He suffered the penalty for all of humanity's sins.

Other men have been crucified; other men have sacrificed their lives on behalf of another; no one else has borne the pains & penalties of all men. No mere man could.

Disclaimer: my thoughts are the product of my own study and do not constitute official statements by The Church of Jesus Christ of Latter-day Saints

I’ll provide a few verses. I’m coming from a classical trinitarian perspective here.

Psalm 62:1-2 (ESV) says:

For God alone my soul waits in silence; from him comes my salvation. 2 He alone is my rock and my salvation, my fortress; I shall not be greatly shaken.

Note that this says that God ALONE is the rock and salvation. So for Jesus to save, he must be God.

Isaiah 43:11 says:

I, I am the Lord, and besides me there is no savior.

Ephesians 2:8-9 says:

8 For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, 9 not a result of works, so that no one may boast.

These verses, combined with Acts 4:11-12, gives a strong case that God is our only savior, and since Jesus is recognized as the savior, he is God. This is true by logical deduction.

11 This Jesus is the stone that was rejected by you, the builders, which has become the cornerstone. 12 And there is salvation in no one else, for there is no other name under heaven given among men by which we must be saved.”

1. If X is the source of salvation, X must be God.
2. Jesus is the source of salvation.
3. Therefore, Jesus must be God.

Given that this conclusion logically follows from 1 and 2, the only way to refute this is to say that 1 or 2 is false. Which is going to be a very difficult task given the previous verses.

Note: I get that this question is referring to the idea that men can’t save. I still included the last syllogism based on the context surrounding the question and the importance of this debate.

Appendix - Jesus as the source, not just the giver.

In line with recent arguments presented by Steve Owen and One God the Father, I'd like to add this end point on the curtails of the syllogism. Notice that the syllogism has now been updated.

Steve and OGF have rightly pointed out that someone could be the giver of salvation (ex: some sort of mediator of salvation, just like Joe from the DMV gives me my license to drive, though he does not create the driving laws) without being the ultimate source. This is a good objection, so we need to consider the verses that identify Jesus as the primary source of salvation. Most pressing, Hebrews 5:9:

And being made perfect, he became the source of eternal salvation to all who obey him, (ESV)

OGF responds to this by citing Young's Literal Translation, which calls Jesus "a cause":

and having been made perfect, he did become to all those obeying him a cause of salvation age-during, (YLT)

This seems to give the Unitarian grounds for interpreting Jesus as merely an employee of God, giving out salvation licenses (keeping with our DMV example here). However, this seems like cherry picking. The following translations identify God as "the source" or "the author" (I've included names for lesser known translations):

NLT, ESV, BSB (Berean Study Bible), BLB (Berean Literal Bible), KJV, NKJV, NASB, NASB 1995, NASB 1977, Amplified, CSB, HCSB, ASV, GNT (Good News Translation), ISV, NAB, NET, NRSV (New Revised Standard Version), NHEB (New Heart English Bible), WNT (Weymouth New Testament, which interestingly says "source and giver"), WEB (World English Bible), and many others, those were just the modern ones put on the Bible Hub Website.

Also note that I left out translations that identified Jesus as "the cause". While "cause" in this case could be identified as synonymous with "author/source", I felt it came to close to the YLT. Even so, it was a small number of translations, and the vast majority (as shown above) use author or source.

So how might the Biblical Unitarian respond to this? I've seen one way and I'll anticipate another, then I'll refute both objections with the same counterpoint. Either the BU will claim that these translations reflect trinitarian leanings, or that Jesus still should only be considered a proximate source, not the real source.

Both of these are refuted by examination of the Greek word used for source. "aitios" is an adjective in the nominative masculine singular that means (according to Strong's encyclopedia) "The cause, author; the culprit, the accused; the crime. From the same as aiteo; causative, i.e. a causer." So even in the Greek, it is clear that this is not proximate, nor a Trinitarian twisting. To claim proximity is ad hoc and should be rejected.

Wow, I just wrote an appendix longer than my original question. Nice :)

The need for and the very fact of the salvation of man is a demonstration of, a revelation of, the righteousness of God:

For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek. For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith. - Romans 1:17

Two related questions pertaining to the nature of this salvation as regards it's mechanism are asked after: 1) Must the Savior be a human, and 2) May the savior be merely human. Most specifically, facet 2 is in question but a short proof of the first facet is in order to bring us to the second.

The first facet is most clearly expressed in Scripture using the language of righteousness. Romans 5 is correctly brought into view by OP as Paul continues to reveal God's righteousness in the Gospel.

• v. 12-14 Sin and it's consequent death enter the world through one man and pass to all man through that one.
• v. 15 The free gift is not like the trespass...one trespass brought death to many but the grace of God in Christ abounded the free gift to many.
• v. 16 The free gift is not like the result of the trespass...judgement followed one trespass unto condemnation but a free gift followed many trespasses unto justification.
• v. 17 Death reigns over man through the offence of one but man reigns in life through the righteousness of one.
• v. 18-19 One man's sin made many men sinners and therefore brought condemnation to all and one man's righteousness justified many and brought life.

This passage in Romans has much more to say about the Righteousness of God in dealing with sin than it does about the constituent natures of the two men it juxtaposes. Nowhere is it stated in this passage that Adam was a mere man. That information must be brought in (rightly) from elsewhere. And nowhere is it stated that Jesus Christ was a mere man or needs to be. Adam was a flesh and blood human and so was Jesus Christ.

Romans 5 explains that one sinless man sinned and the righteous reversal of this is one sinless man remaining sinless. Through one man sin entered and through one man sin is put away. Because God is righteous and just there must be a kinsman-redeemer; there must be one who is fundamentally identified with those being redeemed:

Then cometh Jesus from Galilee to Jordan unto John, to be baptized of him. But John forbad him, saying, I have need to be baptized of thee, and comest thou to me? And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness. Then he suffered him. - Matthew 3:13-15

And so, in order to redeem fallen humanity, one fundamentally identified with humanity needed to come. One born of a woman (as are all men), one born under the law to redeem those under the law (for had Jesus come without the law there would be no identification with those held under it), one unfallen and yet identified in a baptism of repentance with those who are ... all this and more in order to fulfill all righteousness.

The second facet is most clearly revealed in Scripture using the language of debt.

What is owed by the creation to the Creator by virtue of the fact of creation is obedience. Creation is supposed to declare the glory of God. The Creator sustains His creation and nothing is owed by the Creator to the creation as payment for the required obedience for His mercy falls on the just and the unjust:

But which of you, having a servant plowing or feeding cattle, will say unto him by and by, when he is come from the field, Go and sit down to meat? And will not rather say unto him, Make ready wherewith I may sup, and gird thyself, and serve me, till I have eaten and drunken; and afterward thou shalt eat and drink? Doth he thank that servant because he did the things that were commanded him? I trow not. So likewise ye, when ye shall have done all those things which are commanded you, say, We are unprofitable servants: we have done that which was our duty to do. - Luke 17:7-10

But there are wages owed by the Creator to the creation for disobedience:

For the wages of sin is death - Romans 6:23a

Sin creates a debt or deficit of righteousness in relation to God. Obedience is owed and the opposite is offered. This is seen clearly in the Lord's prayer:

And forgive us our debts, as we forgive our debtors. - Matthew 6:12

And especially clearly in Jesus' parable of the unmerciful servant.

One sin is enough to indebt us to God and incur a wage of death (the soul that sins it shall die) but it also appears that this debt is cumulative as suggested by the fact that the King in the parable took account of all his servants. That he found one with a massive debt indicates there were others found with other amounts. Also leading to this conclusion are the servants variously beaten with few stripes or many stripes:

And that servant, which knew his lord's will, and prepared not himself, neither did according to his will, shall be beaten with many stripes. But he that knew not, and did commit things worthy of stripes, shall be beaten with few stripes. For unto whomsoever much is given, of him shall be much required: and to whom men have committed much, of him they will ask the more. - Luke 12:47-48

And the summary statement of Jesus regarding the woman who anointed his feet makes little sense apart from a notion of accumulated debt:

Wherefore I say unto thee, Her sins, which are many, are forgiven; for she loved much: but to whom little is forgiven, the same loveth little. - Luke 7:47

The principle of righteousness states that some man must remove another man's introduction of a foreign disposition into humanity. God is righteous...a man did it so a man must undo it. Focusing on this aspect alone it is perhaps possible that a merely human and perfect Jesus could reverse the effects of a merely human Adam if what were being considered were simply a reversal in principle. But sin also incurs debt and what is not possible is for a mere man to give God a ransom sufficient to pay the accumulated debt of every human being who ever lived. It is this understanding that lies behind the following from Psalm 49:

Truly no man can ransom another, or give to God the price of his life, for the ransom of their life is costly and can never suffice, that he should live on forever and never see the pit. - Psalm 49:7-9

If a mere man can not pay the accumulated debt owed to the Creator for even one other man's life it is far beyond impossible for a mere man to pay the accumulated debt of all men.

The language of debt is carried all the way through to Christ's last words: "it is finished". The debt which no mere man could repay is discharged by someone greater than a mere man who has come. Someone whose own life is more valuable than the sum of all other lives because He is the source of all life. Someone whose life is a more than sufficient ransom for all. He was with God in the beginning and He was God.

The Word was made flesh and dwelt among us we have been told. Interestingly was made is middle deponent in voice (which indicates that the subject performs the action, instead of receives it) and dwelt among us is active in voice (which also indicates that the subject performs the action, instead of receives it). The Word who was God made Himself flesh and caused Himself to dwell amongst us.

The only ministry we have is the ministry of God's reconciliation:

And all things are of God, who hath reconciled us to himself by Jesus Christ, and hath given to us the ministry of reconciliation; To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation. Now then we are ambassadors for Christ, as though God did beseech you by us: we pray you in Christ's stead, be ye reconciled to God. For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him. - 2 Corinthians 5:18-21

It is not that God was through or by Christ reconciling the world but it is that God he was in/to to Christ. A mere man could not do it.