I believe this whole passage is the Holy Spirit telling us to respect all who preach and teach Christ Jesus, being careful not to undermine or discourage.
The Beast of Revelation, the Anti Christ, is referenced in the following:
'That day' is the Day Of The Lord Jesus Christ.
The Falling Away will happen as a result of ➛ false teaching leading to false expectations ➛ deceitfulness of sin and its legitimization ➛ turning from the Word and fellowship in the Holy Spirit.
The 'man of sin' is the beast, the antichrist. The revelation of the beast is accompanied by ➛ a complete disavowal of sin ➛ denial of Jesus Christ ➛ glorification of humanity.
See Dan 7:1-*. Rev 13:1-18. Rev 17:7-14. .