The New Testament mentions a “great apostasy” or “falling away” from the faith that would happen in the last days before Jesus’ Parousia. This prophecy is found primarily in Matthew 24:10-12, but also in 1 Timothy 4:1; 2 Timothy 4:3-4; Jude 17-19. (Some people also include 2 Thessalonians 2:2:1-12 about the Great Apostasy, though this passage may be speaking of the Man of Lawlessness, rather than the Great Apostasy.)
Has this event been fulfilled? We preterists are persuaded that, indeed, it was fulfilled in the first century. Let me demonstrate.
In Matthew 24:34, Jesus said that ALL the things He prophesied would happen in his own generation. (cf. Luke 21:22.) That should settle the issue, at least if you believe Jesus was a true prophet.
But there are other confirmations that the Great Apostasy did occur during the generation of the first Christians.
- Acts 20:29-30 warns about those who are drawing the faithful away by false teaching.
- Romans 16:17-18 indicates a group of people who were causing division.
- Galatians 1:6 mentions people who were deserting the true gospel.
- 2 Thessalonians 2 mentions some being deceived as Paul was writing.
- 1 Timothy 1:18-20 documents some who were “made shipwreck” of their faith.
- In 2 Timothy 3:1-9 Paul warns Timothy to avoid certain bad actors in the “last days.” These people seem to have been associated with those who succumbed to the Great Apostasy. First of all, the culmination of none of the mentions of the biblical “last days” in the New Testament can be placed outside of the first century. The last days were the end of the Old Covenant Age in AD 70, when Jerusalem and the temple were destroyed—not the end of history. Secondly, Timothy was told to avoid such people. So that narrows the time to that of the life span of Timothy.
- Titus 1:10-14 tells of the same problem in the church.
- Hebrews 3:12 warns of those falling away from the faith.
- 2 Peter 2:1-3 also tells of false prophets who are leading people away.
- 1 John 2:19 tells of those who “went out from” their fellowship.
- Jude 3-25 appears to be mostly about men perverting the faith.
- Revelation 2:1-7 mentions those turning away in the context of things which MUST SHORTLY COME TO PASS (Revelation 1:1-3; 22:6-20).
Further evidence is from an examination of the other things in Matthew 24 that Jesus said would be fulfilled prior to “the end.” It is demonstrable that all of the things on Jesus’ prophecy list have been fulfilled. Gary DeMar, in his book Last Days Madness, does an excellent job covering in detail every aspect of Jesus’ prophecies in the Olivet Discourse. Anyone who wants to go over these things with a fine-toothed comb can do so in DeMar’s very helpful book. In each case it can be verified that the prophecies of the Olivet Discourse were fulfilled in the first century. Below is a short summary.
- gospel preached to the whole world (Matthew 24:14) At least 5 times the inspired apostle Paul proclaimed that this had been fulfilled by the time he was writing―Romans 1:8; 10:18; 16:25-26; Colossians 1:6, 23. (The word “world” in this passage is the Greek word “OIKOUMENE,” which is often translated the Roman world. See Luke 2:1; Acts 11:28; 17:6; 24:5.)
- wars and rumors of wars (Matthew 24:6)—We know of wars and rumors of wars in the first century from general history, including from period historian Tacitus. Tacitus said of the period, “It was full of calamities, horrible with battles, rent with seditions, savage in peace itself.”
- famines (Matthew 24:7)—Four famines occurred during the reign of the Emperor Claudius (AD 41-54). One of them was recorded in Acts 11:28.
- earthquakes (Matthew 24:7)—Notableearthquakes occurred during the reigns of Caligula and Claudius (AD 37-54). See also Matthew 27:54; Matthew 28:2, and Acts 16:26.
- persecution of Christ’s disciples (Matthew 24:9)—This is a well-documented fact in the Bible (Acts 4:3; Acts 4:17; Acts 5:40; Acts 7:54-60; Acts 8:1; Acts 9:1; Acts 11:19; Acts 12:1-3; Acts 14:19; 2 Corinthians 11:24-26; Revelation 1:9) as well as general history (Josephus, Tacitus, etc.).
- flee to the mountains (Matthew 24:16)—According to the early church historian Eusebius, the Christians did in fact flee Jerusalem during the Jewish-Roman War. History does not record any Christians having perished in the siege of Jerusalem. So those who followed Jesus’ instructions and fled were saved from the slaughter (Matthew 24:13). This passage, by the way, further confirms that the tribulation was to be a regional event rather than global. It makes little sense if this prescribed way of escaping the tribulation given by Jesus was an instruction for us 2,000 years later.
- false prophets (Matthew 24:24)—Josephus mentions many false prophets and false messiahs. See also Acts 5:36-37; Acts 13:6; Acts 20:29-30; 2 Corinthians 11:13; 1 Timothy 4:1 (note reference to the “latter times”); 2 Timothy 2:16-18; 2 Timothy 3:13; 2 Peter 2:1-3; 1 John 2:18-19; 1 John 4:1; 2 John 1:7.
- astronomical signs (Matthew 24:29)—In this verse we see the sun and moon being darkened, the stars falling, and the heavens being shaken. It is difficult for many people to get over the idea that this language is not about the end of the physical universe. The Old Testament often uses symbolic language about literal events. At least some of such passages are widely acknowledged as having already been fulfilled (Isaiah 13:9-13; Isaiah 34:4-6; Isaiah 51:5-6; Jeremiah 4:1-28; Jeremiah 15:9; Ezekiel 32:7-8; Joel 2:1-10; Joel 3:15-16; Amos 8:9; etc.). But indeed, literal astronomical signs were reported by various historians in AD 66-70, including Josephus. Nevertheless, this was familiar language to those to whom Jesus was speaking and they would have understood Him to be speaking about national judgment rather than the end of the universe. Note specifically Acts 2:15-21 where Peter quotes, as you will recall, astronomical language of the prophet Joel. Peter placed the fulfillment of that prophecy in his own day.
See also these short articles by Don Preston:
 Gary DeMar, Last Days Madness: Obsession of the Modern Church (Powder Springs, Georgia: American Vision), Fourth revised edition 1999, chapter 6.