I would say that they are. I believe in Jesus because of the witness testimonies to his preaching, miracles, death, and most importantly, His resurrection and ascension into heaven. He is who He said He is.
I believe in Divine Revelation because Jesus revered it (in the OT) and gave the apostles authority to teach what He taught them – and, among the methods they chose were verbal (Apostolic Teaching) and written (Scripture).
So, the two questions really are distinct, and as such, have distinct answers.
Aah… you’re asking whether prophets can be deceived about the Revelation that they receive?
What I said is different from what you’re asking, though: initially, you asked if God protected prophets from committing sins, not whether God protects them from the devil. Again, these are two distinct questions.
I would still answer “no” to the first. But, I would think that the answer to the second is that the Father of Lies would not tell the Truth to the prophets of God, so we could discern an error by its very words.
In any case, we as Catholics believe that Divine Revelation ceased with the death of the last apostle and no further revelation is forthcoming. Therefore, since Jesus (implicitly) accepted the prophets of the Old Testament, we don’t have to worry that they prophesied in error. And, there are no ‘real’ prophets forthcoming, following Jesus, so we don’t have to worry that there will be any authentic prophets who will be tempted after the time that He returned to heaven.