Is it ensure, insure, or assure?
in the religious sense, 1580s, from back (adj.) + slide (v.). Related: Backslider; backsliding (1550s).
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.