One common argument against Sola Scriptura is the fact that "Protestants" are often quite divided. Catholics claim that there are anywhere from 30-60,000 "Protestant" denominations, not to mention a shed load of divided individuals, all claiming the Bible as their source of authority, so clearly scripture is insufficient.
There are a wide range of problems with this argument, starting with the fact it is circular. "Your denominations can't all be right, therefore ours is", is flawed logic. It wouldn't even solve the problem, since instead of interpreting the scriptures outright, Catholics must then interpret their Church's interpretation of the Bible. The result of this is that Catholics are actually as divided within Catholicism as "Protestants" are without.
Furthermore, the number of denominations is massively exaggerated. Take, for example, the meme to the left, which I have seen being used by two separate Catholics on separate occasions (one even directly against me). Notice, first of all, it includes decidedly anti-Christian cults, such as Mormonism and the Watchtower. Given that Mormons have 3 extra scriptures, they obviously don't believe Sola Scriptura.
To top all of this off, even within the denominations you have left once you've removed all the obvious outliers, the differences are minimal. They can be so small that they come down to when, exactly, to take communion, whether or not to baptise infants, whether to meet on a Saturday or Sunday, or other silly disputes like this. In fact, the differences are so trivial that members of one denomination (or, in my case, none) can often fellowship in each other's churches. My own congregation is quite diverse. Our pastor is an ex-Anglican, but our church is officially Evangelical, being associated with FIEC. My mother is a Baptist. I refuse, under any circumstances, to declare an affiliation with any denomination. That's a wide range of fancy names for minuscule differences.
But if the Bible is sufficient in and of itself, why do such differences exist in the first place? Put simply, it's because although scripture is sufficient, its readers are not. Catholics would have to agree with this. As previously pointed out, there are divisions within the Catholic Church. In fact, I can tell you, as a man who has debated Catholic priests, Theologians, and many other Catholics, they are divided on almost every issue. Not even salvation is clear in their religion, and you'll struggle to get them to agree on which sources are best to learn their religion from (except, conveniently, themselves). I've seen Catholics debate quite viciously with each other about their own religion. Yet, does the Catholic Church not teach that it is infallible with regard to faith and morals? Then whence commeth the division?
If the Catholic Church can claim they are infallible, yet its members (and even its leaders) are not, then can I, as a Christian, not by the same logic claim that scripture, as the word of God (which Catholicism officially agrees with), is infallible, and yet we, as its students, are not? Any Catholic who refuses to say yes to this question is as beyond hope as a Pharisee. Nothing but being knocked off their horse by Christ Himself can penetrate such a thick skull.
But the question still remains, if scripture is sufficient. why do these divisions occur? Put simply, because the scriptures are not toys, but weapons (e.g. Ephesians 6:17). As Hebrews 4:12 tells us, "...the word of God is living and effective, sharper than any two-edged sword, penetrating even between soul and spirit, joints and marrow, and able to discern reflections and thoughts of the heart." (NABRE). This sounds like a very effective weapon. But like all weapons, practice is required for perfection. No one can expect to pick up a sword and suddenly win every battle they enter. This is why Paul tells us "Be eager to present yourself as acceptable to God, a workman who causes no disgrace, imparting the word of truth without deviation." (2 Timothy 2:15).
The NABRE, which I specifically use for articles aimed at Catholics as it is a popular Catholic translation, is somewhat out of step with other translations here. The word "eager" denotes a sense of joy at the thought of a possibility, which is still good, but nevertheless reduces the strength of the verse a bit. Most other translations, including the Douay-Rheims 1899 American Edition, say something along the lines of study to present yourself as acceptable to God, a workman who needs not be ashamed but rightly divides the word of truth.
Studying and working both require effort. The reason there are so many divisions regarding scripture is because not all put in equal effort, and not all effort produces equal results, much like in every other area of life. Reading Psalm 119, we truly see the benefits of rigorously studying the word of God. In fact, it is this very passage that Bible Brain's tagline "We are the simple the Psalmist talks about" is based on. Anyone can study scripture, even "the simple", precisely because it is sufficient. It is the word of God! In fact, it rather expressly presents its purpose as follows: "But you, remain faithful to what you have learned and believed, because you know from whom you learned it, and that from infancy you have known [the] sacred scriptures, which are capable of giving you wisdom for salvation through faith in Christ Jesus. All scripture is inspired by God and is useful for teaching, for refutation, for correction, and for training in righteousness, so that one who belongs to God may be competent, equipped for every good work." (2 Timothy 3:14-17).
This passage is such irrefutable proof of Sola Scriptura that, in my experience, Catholics just don't deal with it. They'll briefly touch on verse 16's claim of inspiration and say "we believe it's inspired too", but I have never heard a Catholic explain why, if scripture makes us "competent" and "equipped for every good work", why are there so many other, external things the Catholic Church insists scripture isn't sufficient to equip us for, including good works? According to 2 Timothy 3:15-17, scripture:
Makes us wise to salvation
Is useful for teaching
Is useful for rebuke
Is useful for correction
Trains us in righteousness
Makes the man of God competent
Equips us for every good work
Yet, according to official Catholic teaching, scripture, first of all, does not make us wise to salvation. In fact, let's be honest, the Catholic Church's official teachings on salvation are ambiguous and wonky, so much so that even a Catholic Cardinal cannot know, at any given moment, what his eternal fate will be. This is in stark contrast to scripture's claim that those who believe in the name of the Son of God may know that we have eternal life!
But of course, citing that, and many other scriptures that both tell us how to get saved and further assure us of salvation against Catholic teaching is rebuke and correction, is it not? As in, using it for its proper, and explicitly stated purpose. The problem with this is that, officially, Catholics are not allowed to interpret scripture in any other way than they have been told by the Catholic Church. If scripture says one thing, but Catholic tradition teaches another, the Catholic Church is supposed to win.
Among those Catholic teachings are a wide range of "good works" which are completely absent from the Bible. In scripture, there is no Pope, and thus no reason to honor, or even trust such a false prophet. There is no Biblical mandate to confess one's sins to a priest (and indeed, all believers are priests). Purgatory does not exist, and so seeking indulgences or other means to reduce time spent there are oh so obviously unnecessary.
All of this shows us that, even in stark contrast to scripture, Catholicism does not teach the sufficiency of scripture, but it's actually worse than that. See, Catholics, owing to the fact scripture is self-evidently against their religion, must in fact contend that scripture is wrong!
See, if scripture really was a Catholic book, it would read like a Catholic book as surely as other Catholic books, like Catechism of the Catholic Church. It would at least be consistent with Catholicism, as the Catholic Church officially teaches that scripture is equal to "sacred" tradition. At least, by word. In practice, this is impossible, as there are numerous places where scripture and Catholicism contradict. In those places, the Catholic Church basically says "Shut up! We are the authority, we speak for God, you will listen to us, not His word". Not even legitimate Apostles had this much hubris. They encouraged, and even submitted to, the test of scripture, as did Christ Himself. When Catholics follow this command and resist the baseless claims to authority by the Catholic Church, they tend to fall away from the Catholic faith.
The same is true for all errors, because scripture is sufficient. God is the all-sufficient Teacher (Psalm 119:97-100; Matthew 23:8; John 16:13; 1 John 2:27), and so when we become sufficient student to Him, we become united under His truths. When, however, we fail to study under Him, be it through pure laziness, through submission to sin, through devotion to man-made tradition, or through obedience to a heretical Church, we become as scattered as prior to knowing Him.
And so the answer to the Catholic Church's argument is simple. As Mike Winger puts it, "scripture is sufficient, that doesn't mean I am sufficient". Or to put it as I put it, "Protestants" disagree on scripture for the same reason Catholics disagree with scripture. It is a simple case of the Holy God reaching unHoly sinners. Not even the perfect teacher can teach those who cannot, or will not, learn. Therefore, as long as we, as humans, continue to err on anything, we will continue to err on scripture. The question is, will we do so by choice, or by nature? Personally, if I am going to make a mistake with regard to God's word, I want it to be because I tried to listen to Him and misunderstood, not because I ignored Him and listened to a cult instead.