Hi Tim
http://www.giveshare.org/BibleStudy/025.apostate.html
provides a fair answer from a biblical perspective for starters
WHAT IS AN APOSTATE?
We often use "religious" words without knowing their actual meaning. Without thinking, some have used the word "apostate" to refer to anyone who has changed their views on a single doctrine. What is the Bible definition of an apostate?
"Apostate" Defined by the Bible
"Apostate" is translated from apostasia, Strong?s Exhaustive Concordance #646. There are two passages of Scripture using this word:
(1) Acts 21:20-21, " . . . many thousands of Jews there are which believe; and they are all zealous of the law: And they are informed of thee, that thou teachest all the Jews which are among the Gentiles to forsake Moses, saying that they ought not to circumcise their children, neither to walk after the customs."
This was totally untrue, verse 24, as Paul did not teach anyone to apostatize from the law of Moses.
(2) II Thessalonians 2:3, " . . . for that day (the coming of our Lord verse 1) shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition."
There would be a falling away (apostatizing) from the truth, verse 10, NOT an organization. There would be a leader in this apostasy, the man of sin, opposing the Almighty. Apostates are led into strong delusions, that they should believe a lie. They shall be condemned, verses 11-12. The way to avoid apostasy, verse 15, is to stand fast and hold the traditions which you have been taught.
From these two passages, it is obvious that an apostate is one who has utterly renounced the very foundation of the true faith, not just one who adheres to a doctrinal error (heresy), although departure from any part of the truth can and often does lead to total falling away. Under the Old Covenant, physical circumcision was the very seal of the covenant. Rejecting it would be foundational, like rejecting baptism under the New Covenant. The apostate once knew the truth, but now has "pleasure in unrighteousness" II Thessalonians 2:12. He revels in what formerly his conscience forbade him to do. His conscience is seared, I Timothy 4:1-2. He has deliberately renounced his faith and convictions.