On the evening of this Easter Sunday, Jesus appears to the disciples and provides confirmation of his resurrection (20:20). Twice he speaks of “peace” (20:19, 21), fulfilling that which he promised in his farewell: “Peace I leave with you; my peace I give you” (14:27; 16:33). Seeing the Lord was also a part of this promise (16:16; thus for Mary, 20:18; the disciples, 20:20, 25; and Thomas, 20:25, 29), as was rejoicing (16:20; 20:20). In other words, Jesus is recalling his words from the upper room, and this must necessarily include the coming of the Spirit (20:19–29).
That this is a definitive gift of the Spirit…
Baker Publishing Group, The Baker Illustrated Bible Commentary, by Gary M. Burge