first a great apostasy which is false teachings coming from the truth. this is a composite sign. so it is not just one but a group. apostasy means defector but they will do more then leave the teachings of God. they will also commit lawlessness against Jehovah God they will be guilty of treason since they will carry on a religious rebellion promoting false teachings.
2 Thessalonians 2:1 explains itself. That day will not come until rebellion occurs and the man of lawlessness is revealed. Read the verses 1 thru 12. Realize that the church (true followers of Christ Jesus) will be under great attempts to be deceive. Many will fall away. WE MUST KNOW THE TRUTH. Don't be deceived by the man of lawlessness.