What is the meaning of "sola scriptura"?

To put it simply, the words mean : Sola- Alone, solo Scriptura- Scripture It is a Latin phrase for “Scripture Alone”.

What it means- It is the view that Scripture Alone is the Word of God. And that the basis for living and what is righteous living comes from Scripture alone. It is the sole source of authority. Sola Scriptura is one of the pillars of the Protestant Reformation, which all Orthodox Churches hold to today.

Contrast the Roman Catholic teaching that The Church’s words (visa vie the Pope) has greater authority than Scripture. Contrast some charismatic churches that believe that “words of prophecy” or “Special Revelation” are as authoritative for the church as Scripture.

This view began as a defense of the authority of Scripture from other teacher’s opinions.

OP

What is the basis for not believing sola scriptura? Sola scriptura is Latin for "by scripture alone". ... the Bible contains all knowledge necessary for salvation.

Sola Scriptura and the Biblical Canon The basic argument of Sola Scriptura is that all Christian teachings and traditions are to be founded in and subservient to the Bible, and in nothing else.

ANSWER

There's no contradiction there, unless one mixes the idea of the point. The point of sola scriptura is that the bible contains all things necessary for salvation as to faith and practice.

Any teaching and tradition (faith and practice) necessary for salvation are in the bible.

Obviously there are other sources of information and beliefs, but in terms of salvific information, all is in the bible. To say otherwise is to imply that Jesus didn't teach the apostles correctly or that the apostles failed to transmit all the necessary information or that the Holy Spirit failed to inform them subsequent to the ascension.

Or it may be that some who practice Tradition as they see it (obviously the Eastern Orthodox and Roman Catholic vastly differ in this), believe their Tradition is necessary for salvation, but sola scriptura would deny that, if their Tradition is not in the bible or worse contradicts the bible.

So, there is no contradiction in the two statements. They are using different words to describe the same thing. The bible alone is the source of all things salvific and nothing has been left out.

There is, indeed, a modification to the Sola Scriptura stance of the Reformers. You quote one such modern take on it which means that either it is true, or the original stance is true, but they both cannot be true! You have spotted a crucial difference and it deserves attention. In your comment where you cite Lesley's answer, you spot from that Lutheran quotation a subtle deviation from the original Sola Scriptura.

Alas, it will be so hard for me to pin down WHEN this modification happened, rather than WHY; I shall give the former a near-total body-swerve. But in my quotes, it should become clear that a cerebral approach to Scripture that allows our imaginations to take over is the underlying reason why. Whenever people treat Scripture purely as a book that tells everybody everything to do with salvation and religious forms, then it either becomes a closed book, or a means of producing interpretations that become a form of legalism than the Bible itself warns Christians against.

This first quote is from John Calvin (1509-64), the leading 16th century theologian of the European Reformation, who laid a foundation for biblical studies and systematic theology still widely used to this day. Alas, I have no source for this quote so if anyone can provide it, I shall be grateful:

"No sooner do we gain some slight knowledge of God from looking at the world, than we turn from the true God and set up in his place an imaginary vision from our own brain. We draw the worship of justice, wisdom and goodness away from the fountainhead, transferring it elsewhere."

Now, here is what Martin Luther (1483-1546) said about channels of self-disclosure which God has ordained for man – the Word and the Sacraments. Yet the Word is not to be equated with Scripture nor with the Sacraments, though it operates through them and not apart from them. The ‘Word’ is not the Bible as a written book because:

“The gospel is really not that which is contained in books and composed in letters, but rather an oral preaching and a living word, a voice which resounds throughout the whole world and is publicly proclaimed.”

This Word must be heard. This Word must be pondered:

“Not through thought, wisdom, and will does the faith of Christ arise in us, but through an incomprehensible and hidden operation of the Spirit, which is given by faith in Christ only at the hearing of the Word and without any other work of ours.”

More, too, than mere reading is required:

“No one is taught through much reading and thinking. There is a much higher school where one learns God’s Word. One must go into the wilderness. Then Christ comes and one becomes able to judge the world.”

Likewise, faith is given to those who avail themselves of those outward rites which, again, God has ordained as organs of revelation, the Sacraments. Speaking of God and how he can be ‘found’ in the material creation, Luther made clear that:

“…yet he does not wish me to seek him apart from the Word… He is everywhere, but he does not desire that you should seek everywhere but only where the Word is. There if you seek him you will truly find, namely in the Word. These people do not know and see who say that it doesn’t make sense that Christ should be in bread and wine. Of course Christ is with me in prison and the martyr’s death, else where should I be. He is truly present there with the Word, yet not in the same sense as in the sacrament, because he has attached his body and blood to the Word and in bread and wine is bodily to be received.”

Luther maintained that true Christianity is apprehended by faith channelled through Scripture, preaching, and Sacrament. That is why he avidly promoted the study of Scripture in church and school. In church, the pulpit and the altar must each sustain the other. That is why Luther would create dismay by going on at length about a single point of doctrine. People thought he could be unreasonable to risk severing friendship over one such point, but he equated that with saying it was unreasonable to sever friendship over a single point of strangling one’s wife or child. To deny God in one point is to attack God in all. Right doctrine is only obtained through Scripture alone as disclosed via the Word of God. And Christ is the sole revealer. A Christless approach to Scripture will result in corruptions. Source: ‘Here I Stand’ Roland Bainton, pp224-5 (Lion 1988 reprint, Britain)

Now, back to John Calvin for some quotes of his regarding the sole authority of Scripture:

“They place the authority of the church without the word of God: we annex it to the word, and allow it not to be separated from it.” (Institutes 4.8.13)

By ‘They’ he meant churchmen who claimed that church tradition was an equal source of authority to the Bible. By saying they annex tradition to the word, this meant subjecting tradition to the word and not allowing tradition to stand alone from it.

“…God claims it as his own peculiar privilege to rule us by his laws …it is unlawful to transfer to man what God declares to belong only to himself …this completely cuts off all the power claimed by those who would take it upon them to order anything in the church without authority from the word of God.” (Institutes 4.10.7)

“It is the will of the Lord that we shall depend wholly on his word, and that our knowledge shall be confined within its limits; and therefore, if we lend our ears to others, we take a liberty which he has forbidden, and offer to him a gross insult.” (Commentary on Isaiah 8:20)

I suspect that the earliest and faintest deviations from such a Reformed stance on Sola Scriptura might have begun with the Lutheran branch of Protestantism, long ago, but certainly by the time James Arminius died in 1610 it became clear that some had different interpretations of what the Scriptures meant.

The Arminians made a Remonstrance - or protest - to the Dutch Parliament, to change the Reformed position as set out in the Belgic and Heidelberg Confessions of Faith, with respect to five points. And this might be another clue as to deviations - the importance attached to Confessions of Faith, which seem to expand on what Scripture teaches, the better to clarify, yet they so often lead to divisions.

The National Synod of Dort sat for 154 sessions over a period of seven months, seeking to reconcile the Arminian viewpoint with that stated in the Scriptures, but could not do so. Let me give the last word on Sola Scriptura to Charles Haddon Spurgeon whose declaration brings us back to Luther's view of Christ as the sole revealer of Scripture:

"It is no novelty, then, that I am preaching; no new doctrine. I love to proclaim those strong old doctrines that are nicknamed Calvinism, but which are surely and verily the revealed truth of God as it is in Christ Jesus."

I’m not at all convinced that the two interpretations of Sola Scriptura presented in your question are definitive, let alone which one is a corruption of the original meaning. But before we go any further, it is important to establish what “Tradition” means. One source of information says this under the heading ‘Irenaeus and Tertullian held to sola Scriptura’:

It is with the writings of Irenaeus and Tertullian in the mid to late second century that we first encounter the concept of Apostolic Tradition (tradition handed down in the Church from the apostles in oral form). The word tradition simply means teaching. Irenaeus and Tertullian state emphatically that all the teachings of the Bishops that was given orally was rooted in Scripture and could be proven from the written Scriptures. Both men give the actual doctrinal content of the Apostolic Tradition that was orally preached in the churches. From this, it can be seen clearly that all their doctrine was derived from Scripture. There was no doctrine in what they refer to as apostolic Tradition that is not found in Scripture. In other words, the apostolic Tradition defined by Irenaeus and Tertullian is simply the teaching of Scripture... The Early Church operated on the basis of the principle of sola Scriptura. It was this historical principle that the Reformers sought to restore to the Church. Source: https://christiananswers.net/q-eden/sola-scriptura-earlychurch.html

Here are a few excerpts from an article adapted from a chapter entitled "Sola Scriptura" by Mark D. Thomson in Reformation Theology: A Systematic Summary, edited by Matthew Barrett. What Did Luther Really Believe about Scriptural Authority?

Luther was summoned to an interview with Tommaso de Vio, or Cardinal Cajetan (1469–1534), following the Diet of Augsburg in October 1518. When Cajetan challenged him on the basis of the church’s teaching, Luther insisted, “The truth of Scripture comes first. After that is accepted one may determine whether the words of men can be accepted as true.” [1 Martin Luther, Acta Augustana (1518), WA 2:21.5–6; LW 31:282.]

It is clear that Luther was not dismissing the authority of “the words of men” but rather submitting them to what he regarded as a higher authority, “the truth of Scripture.” Throughout his ministry Luther would cite the fathers and the creeds and even some decisions of the early church councils in support of his teaching, but he did not consider them decisive. Yet they were also more than merely illustrative. Insofar as they faithfully expressed the teaching of Scripture, they were to be regarded as authoritative. Scripture was not the only authority but rather the final authority.

He recognized other subsidiary and contingent authorities, not alongside but under the rule of Scripture, which remained his final authority. These other authorities included not only the church fathers but also a number of significant medieval theologians.

His willingness to reason from the Scriptures (following the example of Paul in Acts 17:2) is evident as early as the famous statement made at the Diet of Worms in May 1521:6

”Unless I am convinced by the testimony of the Scriptures or by evident reason—for I can believe neither pope nor councils alone, as it is clear that they have erred repeatedly and contradicted themselves—I consider myself conquered by the Scriptures adduced by me and my conscience is captive to the Word of God.” [7 Martin Luther, Verhandlungen mit D. Martin Luther auf dem Reichstage zu Worms (1521), WA 7:838.4–7; LW 32:112.]

So for Luther, Scripture itself remained the final authority, but this did not eliminate all appeal to the fathers, the creeds, and the decisions of the church. Reading Scripture is a fellowship activity in which the voices of those who have read before us need to be heard attentively. Source: https://www.crossway.org/articles/what-sola-scriptura-really-means/

Finally, here is an extract from an article on the Lutheran view of Sola Scriptura and Tradition:

Sola Scriptura in Lutheran form is not against tradition per se. While some brands of Christianity might insist that if it’s not in the Bible then it’s not Christian, Lutheran theology understands that a tradition is allowable when (a) it is not contradicted by scripture, (b) it serves a purpose that is scriptural, and (c) it is not enforced as a pre-condition for Christian unity. It is nonetheless possible to assert the principle of Sola Scriptura in a manner similar to the bumper sticker that says: “The Bible Says It, I Believe It, That Settles It.” However, a Lutheran theological approach resists simplification. For Lutheran Christians, reading the Bible does not mean setting aside critical thinking skills. Source: https://lutherantheology.wordpress.com/2011/01/18/a-brief-introduction-to-sola-scriptura/

To answer your question, I agree with Martin Luther who said the truth of Scripture comes first and that Scripture is the final authority. Yes, the Bible contains all knowledge necessary for salvation as it says in 2 Timothy 3:16-17:

All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness; that the man of God may be perfect, thoroughly furnished unto all good works. (KJV but took the liberty of replacing ‘throughly’ with thoroughly)

However, that does not mean we have to disengage our intellects or refuse to acknowledge biblical teachings (traditions) that have been handed down from the early church fathers who were taught by Jesus’ disciples.

I’m now going to track down that bumper sticker.