Reflecting on his contemporary Paul’s theological writings, the apostle Peter writes in 1 Peter 3:15-16:

Bear in mind that our Lord’s patience means salvation, just as our dear brother Paul also wrote you with the wisdom that God gave him. He writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction.”

There it is: “[Paul’s} letters contain some things that are hard to understand.” And God’s people said, AMEN.

Of course, we’re not certain which Pauline teachings Peter had in mind, but it seems like there’s a good chance he was talking about passages like 1 Timothy 2:8-15.