Turning against God by abandoning or rejecting former beliefs. The term refers to a deliberate rejection of the faith by someone who used to believe. It does not refer to not understanding or being mistaken. Apostasy is different from heresy (denial of a part of the faith) and from changing denominations. Also, it is possible to deny the faith, as Peter once did, and reaffirm it later.
Originally, apostasy meant literal rebellion. So, the Jewish people were described as “rebels” against King Artaxerxes (1 Esdras 2:23). Also, Jason was described as a “rebel against the laws” (2 Maccabees 5:6–8). The Old Testament lists many examples of spiritual rebellion:
departure from the law (Joshua 22:22)
forsaking temple worship (2 Chronicles 29:19)
willful disobedience toward God (Jeremiah 2:19)
Isaiah and Jeremiah provide many examples of Israel’s rebellions (Isaiah 1:2–4; Jeremiah 2:19). Israelite kings were often guilty of apostasy:
Manasseh (2 Chronicles 33:1–19)
Amon (2 Chronicles 33:21–23)
In New Testament times, many disciples separated themselves from Christ (John 6:66). Judas Iscariot is the most well-known example. The Greek word where we get apostasy from is only in two passages. The apostle Paul was accused of apostasy for teaching others “to forsake Moses” (Acts 21:21). Apostasy will be significant in the end times (2 Thessalonians 2:3). Christians were warned not to be carried away and tricked by the apostasy to come in the end times before the Lord’s return. This apostasy is because of the rise of a rebellious figure who will be used by Satan to do his work (2 Thessalonians 2:3–12; compare 1 Timothy 4:1–3).