Jim,
Apostasia ("apostasy"; the earliest and most common form is apostasis) and apostatès ("apostate") are nouns, the corresponding verb is aphistèmi (infinitive aphistanai).
Apostasia, Dionysios of Halicarnassus 7,1; Plutarch, Galba 1. LXX (Rahlfs), Joshua 22:22; 2 Chronicles 29:19; 33:19; Jeremiah 2:19; 1 Maccabees 2:15; Josephus, Vita, 43; Antiquities 13,219; Ascents of Isaiah, 2:4; NT, Acts 21:21; 2 Thessalonians 2:3.
Apostatès, Polybius 5,41,6; 5,57,4; Diodorus of Sicily, XV, 18; Plutarch, Romulus 9 (of a fugitive slave); Cimon 10; LXX Numbers 14:9; Joshua 22:16,19; Job 26:13; Isaiah 30:1; Daniel 3:32; 2 Maccabees 5:8; 3 Maccabees 7:3; 1 Esdras 2:17; Odes 7:32. (No NT occurrence).
Whereas the Hellenistic context is more often political treason than religious "apostasy," the religious sense is clear in the LXX (as the instance of Ahaz which I quoted in a previous post shows).