From the Haydock Commentary:
Ver. 25. Not forsaking our assembly. St. Chrysostom understands the assemblies of Christians, where they met to celebrate the divine mysteries. Others expound it of not leaving the faith and communion of the Catholic Church by turning apostates: this is confirmed by the following words: for if we sin wilfully,...there is now left no sacrifice for sins. The Novatian heretics understood no pardon for sins after baptism. St. Chrysostom and others understood no second baptism, wherewith to be cleansed in the same manner as before; but the most probable interpretation, and most agreeable to the text and doctrine of St. Paul, seems to be, that now remained no sacrifice for sins, i.e. no other sacrifice but that of Christ, which the apostate renouncing, by quitting and abandoning his faith, thereby cuts himself off from the very groundwork and foundation of salvation, as long as he continues in his apostacy. So that nothing remains for him but a dreadful expectation of God's just and severe judgments. (Witham)
Ver. 26. If we sin wilfully. He speaks of the sin of wilful apostacy from the known truth; after which, as we cannot be baptized again, we cannot expect to have that abundant remission of sins, which Christ purchased by his death, applied to our souls in that ample manner as it is in baptism; but we have rather all manner of reason to look for a dreadful judgment; the more, because apostates from the know truth seldom or never have the grace to return to it. (Challoner)
And from the RSV-CE (note the heading)
A Call to Persevere
19 Therefore, brethren, since we have confidence to enter the sanctuary by the blood of Jesus, 20 by the new and living way which he opened for us through the curtain, that is, through his flesh, 21 and since we have a great priest over the house of God, 22 let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water. 23 Let us hold fast the confession of our hope without wavering, for he who promised is faithful; 24 and let us consider how to stir up one another to love and good works, 25 not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near.
26 For if we sin deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, 27 but a fearful prospect of judgment, and a fury of fire which will consume the adversaries. 28 A man who has violated the law of Moses dies without mercy at the testimony of two or three witnesses. 29 How much worse punishment do you think will be deserved by the man who has spurned the Son of God, and profaned the blood of the covenant by which he was sanctified, and outraged the Spirit of grace? 30 For we know him who said, “Vengeance is mine, I will repay.” And again, “The Lord will judge his people.” 31 It is a fearful thing to fall into the hands of the living God.