The Great Falling Away

Reading Between The Lines.
1-2 And now, what about the coming again of our Lord Jesus Christ and our being gathered together to meet him? Please don’t be upset and excited, dear brothers, by the rumor that this day of the Lord has already begun. If you hear of people having visions and special messages from God about this, or letters that are supposed to have come from me, don’t believe them. 3 Don’t be carried away and deceived regardless of what they say.
For that day will not come until two things happen: first, there will be a time of great rebellion against God, and then the man of rebellion will come—the son of hell. (2 Thess 2:1-3, TLB)
Let no one deceive you by any means; for that Day will not come unless the falling away comes first, and the man of sin is revealed, the son of perdition… (2 Thess 2:3, NKJV)
Let no one deceive or beguile you in any way, for that day will not come except the apostasy comes first [unless the predicted great falling away of those who have professed to be Christians has come], and the man of lawlessness (sin) is revealed, who is the son of doom (of perdition) … (2 Thess 2:3, AMPC)
3 Do not let anyone fool you. For the Lord will not come again until many people turn away from God. Then the leader of those who break the law will come. He is the man of sin. (2 Thess 2:3, NLV)
Sometimes hearing something in different ways brings out the full meaning of the things described. In these verses written by the Apostle Paul and given to the Thessalonians, he is trying to assure them that the Rapture and the Second coming haven’t occurred. But what is the apostasy Paul mentions? Is this something new for these believers?