The Great Falling Away, "Great Apostasy"
"The great falling away is not a general falling away from the Christian faith that occurs in each generation. It is the specific, unique falling away that occurs in the last generation that ushers in the second coming of Christ. There are many New Testament passages that speak of the great falling away."
https://shortbiblestudies.com/what-is-the-great-falling-away/
"The Great Apostasy refers to a significant falling away from the Christian faith. This isn’t a rebellion against the government by unbelievers, this is a rebellion against Christian belief and practice, by previously believing and associating people. This appears to be the near consensus of most reformed and evangelical scholars."
https://ca.thegospelcoalition.org/article/10-things-you-should-know-about-the-great-apostasy/
"Also, Paul says, the “falling away” or “apostasy” (NASB) would come before the day of the Lord. Many have understood from this statement that before Christ returns many people will reject the faith and rebel against God.
Paul uses the Greek word apostasia in 2 Thessalonians 2:3, so “apostasy” is a good, literal translation. The Greek word means “to be apart from or to stand apart from.” Paul uses the same word in 1 Timothy 4 when he explains that in the last days people will stand apart from (or fall away from) the faith and follow various kinds of falsehood (1 Timothy 4:1–3). So, there will be a standing apart from the faith or a great falling away before Christ returns."
https://www.gotquestions.org/great-falling-away.html
"In 2 Thessalonians 2:3, Paul makes it clear that the day of the Lord, a time of worldwide judgment (Isaiah 13:6; Obadiah 1:15), will not transpire until two things happen. First, the falling away, or great apostasy, must occur. Second, the “man of lawlessness” must be revealed, he who is called the “son of perdition,” also known as the Antichrist. Once this person makes himself known, the end times will indeed have come"
https://www.gotquestions.org/great-apostasy.html