Apostasy
2 Thess 2:3
Let no one deceive you by any means; for that Day will not come unless the falling away comes first, and the man of
sin is revealed, the son of perdition
NKJV
Heb 6:4-6
For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become
partakers of the Holy Spirit, 5 and have tasted the good word of God and the powers of the age to come, 6 if they fall away,
to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.
NKJV
The word being used in verse 6 above, or in 2 Thes. is the word used for Apostasy;
Fall away; parapipto (par-ap-ip'-to); to fall aside, i.e. (figuratively) to apostatize:
APOSTASY. A "falling away." The common classical use of the word has to do with a political defection (Gen 14:4, LXX; 2 Chron 13:6,
LXX; Acts 5:37). In the NT its more usual meaning is that of a religious defection. This is called "apostasy from the faith"
(apostasia a fide): a secession from the church, and a disowning of the name of Christ.
(From The New Unger's Bible Dictionary. Originally published by Moody Press of Chicago, Illinois. Copyright (c) 1988.)
Apostasy is generally defined as the determined, willful rejection of Christ and His teachings by a Christian believer
(Heb 10:26-29; John 15:22).
(from Nelson's Illustrated Bible Dictionary, Copyright (c)1986, Thomas Nelson Publishers)
(Heb. meshuwbaah‚; Gk. parapiptoo, aphisteemi, apostasia). Defection from the faith, an act of unpardonable rebellion against God
and his truth. The sin of apostasy results in the abandonment of Christian doctrine and conduct.
(from Evangelical Dictionary of Biblical Theology. Copyright 1996 by Baker Books. All rights reserved. Used by permission.)