How do we know apostasy is real according to the Bible?

Apostasy, while not named explicitly in Scripture, is a theme woven throughout the Bible indicating that not all who appear to be in the faith truly are. Hebrews 3:12-13 warns us to take care lest there be an evil heart of unbelief causing a departure from the living God. Additionally, 1 John 2:19 states that those who leave the faith never truly belonged to it, as true believers will endure. Historical accounts, such as those of Israelite rebels in the wilderness, show that witnessing God’s miracles does not guarantee faithfulness. Apostasy is rooted in unbelief, where one turns from reliance on Christ to self, illustrating the grave danger posed to the genuine faith of both individual believers and the church as a whole.