To draw back or apostatize in matters of religion (Acts 21:21; 2 Thessalonians 2:3; 1 Timothy 4:1). This may be either partial (Proverbs 14:14) or complete (Hebrews 6:4-6; 10:38, 39). The apostasy may be both doctrinal and moral.

Noah Webster's New International Dictionary of the English Language

(v. i.) To slide back; to fall away; esp. to abandon gradually the faith and practice of a religion that has been professed.