2 Thessalonians 2:5-6

v5: Paul had already told the Thessalonian Christians that Christ would not return until this apostasy followed by the revelation of the man of lawlessness had taken place.  They should not have been surprised to hear this (that some believers would depart from the faith), because in the history of God's people, after any significant advance, a defection followed. "For example, soon after people began to call on the name of the Lord, everybody corrupted their ways. Soon after the covenant with Noah, the Babel builders defied heaven; soon after the covenant with Abraham, his descendants became degenerate in Egypt. Soon after the Israelites were planted in Canaan, when the first generation had died, they abandoned God and served Baal. Soon after God’s covenant with David, his descendants rebelled and served other gods; soon after the return from exile there was a general decay of godliness, as can be seen from the accounts of Ezra and Nehemiah. Therefore, it was in no way strange that after Christianity had been planted there would come a general falling away."

v6: At present God, through the power of His Spirit, is holding back the man of lawlessness, this antichrist figure, who will one day appear.  He will continue to do this, stopping the lawless one's wickedness from coming to full fruition, until it is time for him to be revealed and only then will He remove His hand of restraint.