John Calvin Commentary 1 Corinthians 5:3

"For I verily, being absent in body but present in spirit, have already as though I were present judged him that hath so wrought this thing," — 1 Corinthians 5:3 (ASV)

I truly, etc. As the Corinthians were failing in their duty, having condemned their negligence, he now shows what ought to be done. So that this stain may be removed, they must cast out this incestuous person from the community of believers. He then prescribes, as a remedy for the disease, excommunication, which they had sinfully delayed for so long.

When he says that he had, while absent in body, already determined this, he severely rebukes in this way the negligence of the Corinthians, for there is an implied contrast here. It is as if he had said: “You who are present ought to have applied a remedy for this disease by now, having it every day before your eyes, and yet you do nothing; while I, for my part, even though absent, cannot endure it.” To prevent anyone from alleging that he acted rashly in forming a judgment from such a great distance, he declares himself to be present in spirit, meaning by this that the line of duty was as clear to him as if he were present and saw the matter with his own eyes.

Now it is important to observe what he teaches regarding the method of excommunication.