Jamieson, Fausset and Brown on Hebrews 10:26

26. Compare on this and following verses, He 6:4, &c. There the warning was that if there be not diligence in progressing, a falling off will take place, and apostasy may ensue: here it is, that if there be lukewarmness in Christian communion, apostasy may ensue.

if we sin--Greek present participle: if we be found sinning, that is, not isolated acts, but a state of sin [Alford]. A violation not only of the law, but of the whole economy of the New Testament (He 10:28, 29).

wilfully--presumptuously, Greek "willingly." After receiving "full knowledge (so the Greek, compare 1Ti 2:4) of the truth," by having been "enlightened," and by having "tasted" a certain measure even of grace of "the Holy Ghost" (the Spirit of truth, Joh 14:17; and "the Spirit of grace," He 10:29): to fall away (as "sin" here means, He 3:12, 17; compare He 6:6) and apostatize (He 3:12) to Judaism or infidelity, is not a sin of ignorance, or error ("out of the way," the result) of infirmity, but a deliberate sinning against the Spirit (He 10:29; He 5:2): such sinning, where a consciousness of Gospel obligations not only was, but is present: a sinning presumptuously and preseveringly against Christ's redemption for us, and the Spirit of grace in us. "He only who stands high can fall low. A lively reference in the soul to what is good is necessary in order to be thoroughly wicked; hence, man can be more reprobate than the beasts, and the apostate angels than apostate man" [Tholuck].

remaineth no more sacrifice--For there is but ONE Sacrifice that can atone for sin; they, after having fully known that sacrifice, deliberately reject it.