This passage outlines a basic sequence of events: Christ's return is preceded by the appearance of the man of sin, who is preceded by the “falling away.” As we look forward to that Day of both darkness and light, we should watch for this falling away to help us stay focused on the surety of Christ's coming.

We are immediately presented with a challenge, however, because the Bible contains few supporting scriptures to help us understand exactly what Paul means. The Greek word translated as “falling away” is apostasia, which Strong's Greek Dictionary defines as “defection from truth.” This word is used in only one other place:

When he had greeted them, he told in detail those things which God had done among the Gentiles through his ministry. And when they heard it, they glorified the Lord. And they said to him, “You see, brother, how many myriads of Jews there are who have believed, and they are all zealous for the law; but they have been informed about you that you teach all the Jews who are among the Gentiles to forsake Moses, saying that they ought not to circumcise their children nor to walk according to the customs. (Acts 21:19-21)

The word apostasia is found in verse 21, in the mention of “forsak[ing] Moses.” A rumor was circulating that Paul was teaching people to fall away from—to defect from—the truth revealed through Moses, which underscores Peter's observation about how easily people could misunderstand Paul's teachings (II Peter 3:16). Acts 21 gives us a second reference for the usage of apostasia, but it tells us little about what Paul had in mind when he warned of the falling away—a specific and definitive falling away.

A closely related form of this word is apostasion, which means “separation.” In the three places it is used, it is translated as “a certificate of divorce” (Matthew 5:31; 19:7; Mark 10:4). In that related form as well, we see the concepts of falling away, forsaking, and defecting.

Apostasia is also found in four places in the Greek translation of the Old Testament, the Septuagint (Joshua 22:22; II Chronicles 29:19; 33:19; Jeremiah 2:19). In each case, the predominant theme is a king (or the whole nation of Israel) rebelling against—forsaking—God and turning to an alien belief system. Applying that to the prophesied “falling away,” we can understand that the truth forsaken in the end times is not truth as an abstract concept, but truth that relates to God.