The great apostasy is in the study of eschatology, i.e. the coming of the Lord and end of the age, etc. Both Jesus and Paul predicted the Apostasy. The reason this study is so important is because the apostasy was to be a sign of the coming of the Lord — when the apostasy was at its height the Lord was to come, II Thessalonians 2:2ff.
How prevelant was the apostasy to be? In Matthew 24:10-12 Jesus indicated it would be universal! In verse 10 he said "many will be offended" because of persecution. In verse 11 he said false prophets would "deceive many". Then in verse 12 he said because lawlessness shall increase "most people’s love will grow cold" [NASV]. Notice the Lord said "most" people would lose their faith. That is an awesome thought — an awesome apostasy.
Luke 18:8 supports this. Jesus asked: "When the Son of Man comes will he really find faith on earth?" The force of the Greek is that Jesus was saying there would be few indeed who remained faithful. The majority of Christians then, would leave the faith because of persecution, false teachers, lawlessness, etc. The question is did the apostasy take place; or is it future?
In 2 Thessalonians 2:7 Paul said that the spirit of lawlessness was at work as he wrote. In Matthew 24:34 Jesus unequivocally said the events he was predicting would occur in that generation. Now many attempt to redefine the word generation to mean "race of people" or project the generation to the distant future. But this is a violation of the meaning of the word and context. When the term "this generation" is used in the Bible it never refers to a specific race of people and never refers to the distant future — it refers to the people all living at that time. The apostasy was to occur in Jesus’ generation. The New Testament record bears this out.
Romans deals with major problems within the body, Romans 16:17f. Consider Corinth. Paul, writing to all the churches of Galatia said "I marvel that you are so soon removed from him who called you". Colossians deals with Judaizers defrauding Christians of their faith and reward. Philippians warns of "the enemies of the Cross". All familiar with Thessalonians know of Paul’s deep concern for their faith. Paul instructed Timothy, in two epistles, to deal with the ungodly who would destroy the faithful. Hebrews is an apologetic to stem the tide of those "once enlightened" but now falling away. Jude says the scoffers predicted by Jesus and the other apostles were present. John lamented that "many false prophets are gone out into the world" and spoke of many who had "gone out from us".
The unbroken testimony of the epistles is one of the inspired writers attempting to stem the tide of apostasy and encouraging faithfulness. The Great Apostasy happened in the first century generation just as Jesus said it would! How much clearer could the Bible tell us it had happened? Since the apostasy occurred as Jesus predicted and it was to be a sign of the coming of the Lord, then the coming of the Lord was to happen in that generation as well — if not why not?