“Now Enoch, the seventh from Adam, prophesied about these men also, saying, “Behold, the Lord comes with ten thousands of His saints, to execute judgment on all, to convict all who are ungodly among them of all their ungodly deeds which they have committed in an ungodly way, and of all the harsh things which ungodly sinners have spoken against Him.” – Jude 14-15
St. Jude draws upon 1 Enoch again here; this time, though, is an even stronger reference, as he mentions (and attributes the quote to) the Prophet Enoch by name and then goes on to offer a word-for-word quote from 1 Enoch. This verse forces the Sola Scripurist into a catch-22 position. St. Jude clearly believes that this verse comes from none other than “Enoch, the seventh from Adam,” himself, however, we know that 1 Enoch was written no earlier than about 300 BC, well after the lifetime of the historical Enoch, and so, obviously, could not have been written by Enoch. There’s only two possible options that remain, either 1. St. Jude mistakenly believed that Enoch was the author of 1 Enoch and passed on this erroneous belief in this letter (which position conflicts with Sola Scriptura, as Sola Scriptura relies on a belief in absolute Biblical inerrancy), or 2. this verse (and others from the same book?) is part of a millennia-old oral tradition passed down from the Prophet Enoch but not written down until about 300 BC. Both options are incompatible with Sola Scriptura, but they’re the only options we have. Either St. Jude was mistaken and the Bible lacks the absolutist version of inerrancy which Sola Scripturists must hold or St. Jude was drawing on a several thousand-year-old oral tradition, which means St. Jude was no Sola Scripturist and endorses the remarkable reliability of oral tradition.
And, if they were willing to admit that this prophecy was passed down accurately via oral tradition from Enoch, they must also answer the question of why 1 Enoch is not part of their Old Testament. They could assert the case that only this single verse was a real prophecy of Enoch, but the obvious problems with this are that the entire book claims to be from Enoch and that St. Jude clearly believes it is; as we’ve seen, this quotation isn’t the only time he cites this book.
In the end, there’s a lot of hard questions for Protestants to answer that are raised by this verse from St. Jude’s letter. If St. Jude’s quote here were the only extra-biblical quote we had in Scripture (and it’s not, clearly), it would still be enough itself to refute Sola Scriptura completely.
I’d provide examples for comparison, but 1 Enoch is, in fact, the only “Old Testament” book that St. Jude quotes.