You asked if Paul contradicts John, and I said no, they both teach the saints can forfeit salvation. Now you want proof, so here you go:
In Romans 6:16, Paul says to the saints who choose to give sin dominion over themselves that it's "unto death".
In 1 John 2: 24, John says, "Let that therefore abide in you, which ye have heard from the beginning.
If that which ye have heard from the beginning shall remain in you, ye also shall
continue in the Son, and in the Father."
Now, 11th hour came up with that kooky idea that Paul's "unto death" ain't referring to
spiritual death
even though it's contrasted with "righteousness", a clearly
spiritual concept. I'm sure you'll have something similar to show how if what we've "heard from the beginning" fails to "remain in" us, we can still "continue" in God, right?