Antichrist must by definition teach what is contrary to the faith. A heretic cannot become Pope - first, because a heretic cannot be elected Pope, second because it is not men who elect the Pope, but the Holy Spirit. In fact, I think that not only he could not be a priest or bishop, but even as a lay person he’d be under excommunication for heresy - that’s if he were born Catholic, which would imply that he was baptized, which probably won’t happen.
Antichrist will be someone who is the opposite of Christ in his actions, and has power do do great signs. This means he will probably despise religion and the concept of a visible body of faithful. I see him more as a powerful agent of chaos, anarchy, and disorder, with a message focused on deep atheism and relativism, yet so convincing and enticing (given also the fact that his moral law will be the opposite of God’s, meaning full satisfaction of the senses even and especially to the injury of others) that many if not most of the people will follow him. But those are only my thoughts.