Backslide

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

Part of speech: verb (v.)

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