It's hard to convince people nowadays there's such a thing as true doctrine and false doctrine. But there's always been truth and falsehood. No one knows 100% of the truth, but there are gross errors that should be obvious to anyone. Peter gives one example in 2 Peter 2:1, "even denying the Lord who bought them." The teaching of transubstantiation is another; it's closely related to Peter's definition. It's a bit ironic that Catholics teach the very thing Peter warned about.
Maybe this one too?
2 Corinthians 11:4
4For if someone comes and proclaims a Jesus other than the One we proclaimed, or if you receive a different spirit than the One you received, or a different gospel than the one you accepted, you put up with it way too easily.