For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,
Compare Hebrews 6:4, etc. There the warning was, that if there be not diligence in progressing, a falling off, and then an apostasy, will ensue: here it is, if there be lukewarmness in Christian communion, apostasy ensues.
If we sin, х hekousioos (G1596) hamartanontoon (G264): present participle] - if we at that day be found sinning; i:e., not isolated acts, but in a state of sin (Alford): sin against not only the law, but the whole Gospel economy (Hebrews 10:28-29).
Wilfully - presumptuously; 'willingly.' After receiving 'full knowledge х epignoosin (G1922): cf. 1 Timothy 2:4] of the truth,' by having been "enlightened," and having "tasted" a measure even of "the Holy Spirit" (the Spirit of truth, John 14:17; and "the Spirit of grace," Hebrews 10:29), to fall away ("sin," Hebrews 3:12; Hebrews 3:17: cf. Hebrews 6:6) to Judaism or infidelity is not an ignorance or error ("out of the way," Hebrews 5:2: the result) of infirmity, but a deliberate sinning against the Spirit: a consciousness of Gospel obligations not only was, but is present: a sinning presumptuously and perseveringly against Christ's redemption for us, and the Spirit of grace pleading in us. 'He only Who stands high can fall low. A vivid apprehension of good is necessary in order to be thoroughly wicked; hence, man can be more reprobate than beasts, and 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 it, deliberately reject it.