You mean the beast of Rev 13 or what?

Beast of Daniel 7?

Paul said the "man of sin" would be revealed after the falling away. He further said the restrainer was in the way.

What happened was pagan Rome as the restrainer fell followed by the rise of papal Rome.

As papal Rome came about the church changed. This was the predicted "falling away" followed by man of sin-antichrist, symbolic for Rome's false Christianity.

There are many identifiers in Daniel 7 and Rev 13 that fit only papal Rome.

The entire notion the antichrist is still to come is a new doctrine.