I quote in part Heb. 10:25, Not forsaking the assemb1ing of ourselves together." The context serves a severe warning against apostasy. One might ask, "What is apostasy?" 1 Tim. 1:19 says it is "making shipwreck of the faith." That doesn't mean to lose faith; that means to go off the deep-end and actually depart from the faith of the gospel as did Hymenaeus and Alexander who (said Paul) I have delivered over to Satan in order that they may be taught not to be blaspheming.
Apostasy is the heart's departure from the living God (Heb. 3:13). It is a "drawing back" (Heb. 10:38), a "turning back (John 6:66). Here in our text it is (at least in its beginning) a forsaking the assembly of believers.
The "forsaking of the assembly" is not merely quitting a religious assembly, but a forsaking the worship of Christ. John Owen said, "the word assembly is put for the whole worship of Christ because worship was performed in their assemblies; and he that forsakes the assemblies, forsakes the worship of Christ." He is not talking here about the man who becomes lazy and slack in his church attendance (though that may be the forerunner); but he is speaking of the man who has abandoned the place where Christ is truly worshipped. That, my friend, is apostasy and could be fatal."