The word for apostasy means abandonment or withdrawal from the truth. And we have always had apostates, however, this is a very specific event that will reveal the Antichrist and his deception.So in effect, The apostle Paul is saying this to comfort the believers of that day who were believing that they were living in the Day of The Lord (which would have meant that they had missed the rapture.) Paul assures them that certain things must come first and since they had not come they are ok-they did not miss the rapture because it had not happened.