While past studies on apostasy in Hebrews have disproportionately focused on the warnings in Hebrews 6:4–12 and, to a lesser extent, Hebrews 10:26–31, I argue that the use of Deuteronomy 29:18–19 in Hebrews 12:15, the last of the book’s five warning passages, is uniquely illuminating to the author’s theology of perseverance and apostasy. This becomes especially clear when Hebrews’ appropriation of the Deuteronomic passage is seen against the background of its reception in the Second Temple period. This study surveys the use of Deuteronomy 29:18–19 in Second Temple Judaism, demonstrating that this passage was uniquely used to furnish the vocabulary for speaking of the dangers of hidden religious hypocrites within the covenant community (1QS II.11–15; CD III.5–9; 1QHa XII.14–17; LAB 25.5–6; Acts 8:21–23; b.Sanh. 76b). This in turn sheds light on Hebrews’ theology of a mixed covenant community, in which apostasy functions as the manifestation of the pre-existing heart of unbelief (Heb 3:12; cf. 3:14; 6:12). I will conclude that Hebrews can be profitably placed in dialogue with other New Testament texts that have a more pronounced doctrine of divine preservation (e.g., 1 Cor 1:7; 1 John 5:3), leading to a nuanced view of the interplay between external community membership, individual perseverance, divine preservation, and apostasy.