Apostasy - apostasia /ap·os·tas·ee·ah) to rise up in open defiance of authority, with the presumed intention to overthrow it or to act in complete opposition to its demands—‘to rebel against, to revolt, to engage in insurrection, rebellion a falling away, defection, rebellion

It reveals a heart condition whose state is unregenerate and in revolt against God.

It demonstrates that one’s commitment to former beliefs where superficial at best but there was no true surrendering to the authority of God in the heart. This is borne out when their faith is put to the test and their true condition is made manifest.

Apostasy should be distinguished from error being taught in ignorance or heretical doctrine where one has an opinion or teaching contrary to scripture.

However, when a person has been clearly shown the heretical teaching they are promoting or following is against the clear teaching of Biblical truth and continues to refuse to submit to sound doctrine, heresy becomes like apostasy due to the fact that both are an act of rebellion against Biblical truth.

Apostasy is an ongoing danger to the church starting from the church's inception until now, The NT contains many warnings against it (Acts 20:29, 1 Tim. 4:1–3; 2 Pet. 3:17). Its' arena of operation is within the walls of the church which makes it particular dangerous because of its inside mode of operation.

In the Old Testament unrepented apostasy had become widespread in Israel has denoted by Jer. 2:19; 5:6; cf. Josh. 22:22; 2 Chron. 33:19.

OT descriptions of spiritual rebellion include departure from the law, forsaking temple worship, and willful disobedience toward God himself (Jos 22:22; 2 Chr 29:19; Jer 2:19). The prophetic writings of Isaiah and Jeremiah provide many examples of Israel’s defections (Is 1:2–4; Jer 2:19). Israelite kings were often guilty of apostasy: Rehoboam (1 Kgs 14:22–24); Ahab (1 Kgs 16:30–33); Ahaziah (1 Kgs 22:51–53); Jehoram (2 Chr 21:6, 10); Ahaz (2 Chr 28:1–4); Manasseh (2 Chr 33:1–19); Amon (2 Chr 33:21–23)

Apostasy is given an eschatological (end times) significance in 2 Thessalonians 2:3. Christians were warned not to be carried away and deceived in the widespread apostasy to come in the end times before the Lord’s return. That apostasy is linked to the rise of a man of rebellion who will be Satan’s tool (2 Thess 2:3–12; cf. 1Tim 4:1–3).