John Calvin on Matthew 18:31-33

31. When his fellow-servants saw what was done. Though we ought not to search for mystery in these words — because they contain nothing but what nature teaches, and what we learn by daily experience — we ought to know that the men who live among us will be so many witnesses against us before God; for it is impossible but that cruelty shall excite in them displeasure and hatred, more especially, since every man is afraid that what he sees done to others will fall upon his own head. As to the clause which immediately follows, it is foolish to inquire how God punishes those sins “Comment il est possible que Dieu punisse;” — “how it is possible for God to punish.” which he has already forgiven; for the simple meaning is this: though he offers mercy to all, yet severe creditors, from whom no forgiveness can be obtained, are unworthy of enjoying it.