Evstevemd:
The man of sin is not a man who becomes Satan. He is Satan all along. He is THE Antichrist. He is against Christ, and always will be. John saw many antichrists in his day. These were the Jews (and possibly Gentiles) who saw Him do all the miracles, heard Him teach, saw Him raise people from the dead, heal the sick, and so forth, yet because of an evil heart, refused to acknowledge that He was the Son of God. They were the many Antichrists. They were the children of disobedience in whom the spirit of iniquity was working. The "spirit of iniquity" was the spirit of Antichrist.

The Antichrist didn't become Satan, he was Satan.