John Calvin Commentary Hebrews 10:27

"but a certain fearful expectation of judgment, and a fierceness of fire which shall devour the adversaries." — Hebrews 10:27 (ASV)

But a certain fearful looking for, etc. He means the torment of an evil conscience that the ungodly feel. They not only have no grace, but they also know that, having tasted grace, they have lost it forever through their own fault. Such people must not only be pricked and bitten, but also tormented and lacerated in a dreadful manner. This is why they war rebelliously against God, because they cannot endure so strict a Judge. They indeed try in every way to remove the sense of God’s wrath, but all in vain; for when God allows them a short respite, he soon draws them before his tribunal and harasses them with the torments they especially shun.

He adds, fiery indignation, or the heat of fire; by which he means, I think, a vehement impulse or a violent ardor. The word fire is a common metaphor; for just as the ungodly are now in a heat from the dread of divine wrath, so they will then burn with the same feeling. It is not unknown to me that the sophists have speculated with great refinement about this fire; but I pay no attention to their interpretations, since it is evident that this is the same way of speaking as when Scripture connects fire with the worm (Isaiah 66:24). But no one doubts that “worm” is used metaphorically to designate that dreadful torment of conscience by which the ungodly are gnawed.

Which shall devour the adversaries. It will devour them in such a way as to destroy them, but not to consume them, for it will be inextinguishable. And so he reminds us that all who have refused to hold the place granted to them among the faithful are to be considered enemies of Christ. For there is no intermediate state, as those who depart from the Church give themselves up to Satan.