What is Apostasy or an Apostate

APOSTASY (ἀπόστασις, apostasis, ἀποστασία, apostasia). A public denial of a previously held religious belief and a distancing from the community that holds to it. The term is almost always applied pejoratively, carrying connotations of rebellion, betrayal, treachery, or faithlessness.

Etymology

The Greek terms ἀπόστασις (apostasis) and ἀποστασία (apostasia), from which “apostasy” derives, typically appear in a political or military context. “Apostasy” is also related to the term ἀφίστημι (aphistēmi). Ancient literature presents two prevalent senses for the terms (e.g., Herodotus, Histories, 3:128; Thucydides, 1.122; Josephus, Vita, 140):

1. rebellion in the sense of disobedience to a king or superior

2. defection during a conflict

The Hebrew terms מרה (mrh), סָרָה (sarah), and מַעַל (ma'al) are used similarly in Old Testament narratives to denote rebellion against a king (e.g., Gen 14:4; Jer 52:3) or against God’s directives (e.g., Num 14:9; Isa 36:5).

Jeremiah 2:19

      19       Your evil will chastise you, 
      and your apostasy will reprove you. 
                  Know and see that it is evil and bitter 
      for you to forsake the LORD your God; 
      the fear of me is not in you, 
                declares the Lord GOD of hosts. 

The Holy Bible: English Standard Version, (Wheaton, IL: Crossway Bibles, 2016), Je 2:19.

Isaiah 1:4-5

      4       Ah, sinful nation, 
      a people laden with iniquity, 
                  offspring of evildoers, 
      children who deal corruptly! 
                  They have forsaken the LORD, 
      they have despised the Holy One of Israel, 
      they are utterly estranged. 
            5       Why will you still be struck down? 
      Why will you continue to rebel? 
                  The whole head is sick, 
      and the whole heart faint. 
            6       From the sole of the foot even to the head, 
      there is no soundness in it, 
                  but bruises and sores 
      and raw wounds; 
                  they are not pressed out or bound up 
      or softened with oil. 

The Holy Bible: English Standard Version, (Wheaton, IL: Crossway Bibles, 2016), Is 1:4–6.