The Christian Duck Test?

Simply because a Christian should understand that there is no salvation by 'works'. All are guilty and deserving of death no matter how much they think they have 'atoned' for their sin.

Only by Christ can a man be saved and receive eternal 'heavenly' life. And his sacrifice covers all men - all will have ample opportunity to accept his payment on their behalf - thus removing the penalty of sin - death.

Some think it's faith plus works. No, works are a result of faith and a necessary result of a living faith, but these works do not 'add' to the salvation already provided. They may well contribute to a 'reward' but that is another matter entirely.

The relationship between God and man has been fundamentally damaged and beyond restoration from our side. God's deepest desire is to restore man to His life and eternal existence. He has provided the means and in time all will be made aware of that provision. For those who know Him now and have a desire to honour and worship Him, they understand that it is God who provides that desire and ability to worship and obey.

For it is God who works in you to will and to act on behalf of His good purpose. Phil 2:13

So God places a measure of His spirit in us to accommodate this new way of living. It does not come naturally as we were at war with Him, being under the lordship of the devil until God changes something in each believers life.

It is His spirit that brings about a change of heart and mind that is not of our doing, but of our acquiescence. Just as Jesus submitted to God's will, so must we in exactly the same fashion.

Intellect has nothing to do with it.

Jesus said, "I praise you, Father, Lord of heaven and earth, because you have hidden these things from the wise and learned, and revealed them to little children. Matt 11:25

The idea that the "duck test" is valid presupposes a philosophical assumption. That assumption is that only the exterior life is valid - that any interior life or attributes, ones not visible to the outside, do not really exist.

There are people who believe this, but Christianity does not admit it. There is enough teaching about the interior life in Christianity to be sure, if Christianity is true, that the inner life is not an illusion.

So the answer to your question is that there is a real difference between one who believes and one who does not, even if that difference is not observable by an outside human observer. The difference is the obvious one - one believes and one does not. It is like the difference between a person who loves their spouse and a person who does not - it is a real difference, even if the latter acts perfectly the part of a loving spouse.

Christianity believes in an omniscient God who can actually observe the inner life, so not only is the difference real it is observable.

That is not to say there should be no difference between the actions of a saved and unsaved person. But it is not the actions that make the difference of salvation - it is the difference of salvation that cause the actions.

Understanding the "duck test"

"If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck."

1. Using the "duck test" is like applying Occam's razor: why do we care whether there is additional element in a Christian's soul (the Holy Spirit) if the external behavior of the whole person is like a Stoic?
2. Alternatively, the "duck test" is like applying the Turing test: if a Stoic can fool people into believing that he is a Christian, then isn't he a Christian for all intents and purposes?

Appropriate test cases for the "duck test"

Important notes for proper test:

1. Obviously successful "duck test" really depends on how good the Turing test is. In other words we have to STRESS TEST THE DUCK in order for the test to be real: trap the duck and put the gun close to the duck's head, then we will see whether it is a REAL duck.
2. Therefore, if we want to "duck test" a Christian properly, we need to include real life situations where the Christian is most likely to behave differently than the Stoic, such as:
   • Response under persecution: see lives of Saints and Martyrs. Do they behave like a Stoic?
   • Response under hardship, like how Job, Abraham, Moses, Daniel, Jesus and St. Paul were tested. Do they behave like a Stoic?

This is very similar to how God let Satan tests Job.

Under stressful situations above, you would see the test fails:

• a martyr empowered with a peace and confidence that is out of this world (observable sign of God's grace and power given through the Holy Spirit in their lives)
• a saint giving up the world to live an ascetic prayer life to commune with God (with one foot appearing to be in heaven already); a life that will seem irrational to a Stoic
• a Christian under hardship (like Job, Moses, and Jesus) would let out their emotion and cry out to God for deliverance, instead of a Stoic resigning to their fate (or resigning to the "duty assigned to them by God") which then allows them to "keep the stiff upper lip" and to shove their emotion under the rug
• a Christian facing an injustice can both be angry at the evil done to them and yet forgive their enemy, like what Jesus and Paul did
• while I do admire a Stoic for their way to deal with suffering, can they rejoice in suffering like a Christian?

CONCLUSION: a genuine Christian under extreme test cases will turn out to be a swan, not a duck.

Answers to your sub-questions

If a Christian and a Stoic both live the same exact life, and the only difference in their essence is that the Christian believes in the gospel, then why does the Christian go to heaven while the unbelieving Stoic does not?

Let's say both the Christian and the Stoic never had any of the hard test cases above, so externally they look the same. You're correct that only those who believe in the gospel can go to heaven, i.e. those who have the Holy Spirit in their hearts.

But I sense a possible misunderstanding when you say "believes in the gospel". A lot of people in the Western world emphasize "head knowledge" too much, but Biblical "belief" needs to include the affective and volitional aspects as well. This means trusting God in difficult situations (think Abraham, Joseph, Jacob, Daniel, Paul) and obeying all Jesus's commandments when it's not convenient. That is why the marriage analogy is the one God chooses for illustration. Like a marriage vow that promises togetherness until death in health and in sickness, in riches and in poor, etc. only those who stay obedient and faithful to Jesus until the very end can enter heaven. Therefore, "believes in the gospel" is much harder in practice than it sounds, but grace makes it possible.

CONCLUSION: A Christian goes to heaven by virtue of what they are after they "believe in the gospel". While a Christian and a Stoic can produce the same deeds under normal circumstances, God looks at the engine, which is different.

When one comes to believe in Christ (esp. when converting from stoicism) what is it that actually changes in the person's essence/soul/existence that makes him fit for the Kingdom of God?

Like what @Spirit Realm Investigator said, what changed is the presence of the Holy Spirit in our soul, who is continually doing work to sanctify us (as long as we cooperate) until we are fit to enter heaven. It is extremely important that we don't delude ourselves into thinking that we are okay. We need to undertake a life of discernment to detect our hidden sins, to repent, and ask for grace continually just like a healthy spousal relationship.

What is it about the intellectual position of being Christian that impacts the state of someone's soul?

As explained above, intellectual position is just one component of "believes in the gospel". A soul also has emotion and will which need conversion. This cannot be stressed enough. I may know a lot about theology (right doctrines), but my intellect can play tricks on me by lowering the standard. Furthermore,

• obeying Jesus commandments that my intellect already choose for me to do in a particular situation and
• taming my desires (who always want to do the contrary)

are EXTREMELY HARD. Only by trying to do the above can we realize how hard it is and that we need God's grace. Grace is what a Stoic does NOT have.

If we assume that the Christian worldview is accurate, then a true Christian has something that a Stoic doesn't: the Holy Spirit.

Acts 1:8 (ESV):

8 But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth.”

The Holy Spirit empowers the Christian to be able to overcome the desires of the flesh and produce the fruit of the Spirit:

16 But I say, walk by the Spirit, and you will not gratify the desires of the flesh. 17 For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do. 18 But if you are led by the Spirit, you are not under the law. 19 Now the works of the flesh are evident: sexual immorality, impurity, sensuality, 20 idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, 21 envy, drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God. 22 But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, 23 gentleness, self-control; against such things there is no law. 24 And those who belong to Christ Jesus have crucified the flesh with its passions and desires. [Galatians 5:16-24, ESV]

The Holy Spirit also distributes supernatural gifts among Christians as He sees fit:

4 Now there are varieties of gifts, but the same Spirit; 5 and there are varieties of service, but the same Lord; 6 and there are varieties of activities, but it is the same God who empowers them all in everyone. 7 To each is given the manifestation of the Spirit for the common good. 8 For to one is given through the Spirit the utterance of wisdom, and to another the utterance of knowledge according to the same Spirit, 9 to another faith by the same Spirit, to another gifts of healing by the one Spirit, 10 to another the working of miracles, to another prophecy, to another the ability to distinguish between spirits, to another various kinds of tongues, to another the interpretation of tongues. 11 All these are empowered by one and the same Spirit, who apportions to each one individually as he wills. [1 Corinthians 12:4-11, ESV]

In light of this, when you say "if a Christian and a Stoic both live the same exact life, and the only difference in their essence is that the Christian believes in the gospel [...]", I say: the condition of your question is ill-defined from the Christian viewpoint, since a Stoic, who lacks the gift of the Holy Spirit, by definition, cannot be living the same exact life as a Spirit-filled Christian. The latter enjoys the supernatural empowerment and assistance of the Spirit of God that the former lacks.

I will close with a pertinent quote from Romans, chapter 8 (ESV):

9 You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him. 10 But if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness. 11 If the Spirit of him who raised Jesus from the dead dwells in you, he who raised Christ Jesus from the dead will also give life to your mortal bodies through his Spirit who dwells in you.

12 So then, brothers, we are debtors, not to the flesh, to live according to the flesh. 13 For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live. 14 For all who are led by the Spirit of God are sons of God. 15 For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” 16 The Spirit himself bears witness with our spirit that we are children of God, 17 and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him.

Answering concerns raised in the comments:

(1) Is the spirit something you can feel within you, and recognize in others? Also, regarding faith, do you "know" when you have faith in Christ? Is it a change in category or a change in degree?

I think these questions can be answered both scripturally and by appealing to the first-hand experience of many believers.

Scripturally, the Bible has lots of examples where believers were filled with the Spirit and had extraordinary experiences. The book of Acts is a good example. That said, a more common --and perhaps less extraordinary-- experience is the "inner witness of the Holy Spirit", which is described very well by Romans 8:15-17 (already quoted above):

15 For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” 16 The Spirit himself bears witness with our spirit that we are children of God, 17 and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him.

Experientially, throughout history many believers have reported personal encounters and extraordinary experiences with the Spirit of God. I have asked questions about this in the past, feel free to take a look: 1, 2, 3.

(2) This answer is self contradictory, since at least some of the fruits of the Holy Spirit listed therein coincide with Stoic virtues, yet the answer goes on to subsequently deny that the Stoics are, at least partially, indwelled by God's Spirit.

Without the Holy Spirit, a Stoic would not be able to achieve 100% of what they otherwise would be able to with the power of the Holy Spirit. But that doesn't mean that they will achieve 0%. It's still theoretically possible for a pagan to develop some moral virtues that would place them somewhere in between the two extremes. Claiming that it's either 0% or 100% would be a false dichotomy, and I never intended to claim such a thing in my answer.

Regarding the salvation status of virtuous pagans, this question might be of interest: What is an overview of Christian viewpoints on the eternal destiny of individuals who die never hearing the Gospel?