What I do thou knowest not now. — Here both pronouns are emphatic, and convey a rebuke to Peter. His words had almost implied that the Lord’s.act was wholly out of place, as of one who knew not what he was doing. The opposite was really the case. “What I do thou knowest not now.”
But thou shalt know hereafter — i.e., in the teaching which is to follow (John 13:13-17). The word rendered “hereafter” is different from that rendered “afterwards” in John 13:36. The precise meaning is “after these things.” The sense, then, is “What I do thou knowest not now; but thou shalt come to know presently.” (Comp. John 13:17.)