CHRYSOSTOM:
The phrase “he remains with you” is what someone who himself is leaving would say. And so, to calm their grief, he says that as long as he remains with them the Spirit won’t come, which means they wouldn’t come to know the greater or more sublime things to come. He wanted them, in other words, to see his departure as a blessing. Notice how he often calls [the Spirit] Comforter, because of the troubles they had to deal with. And since they were still troubled, even after hearing all this, because of their sadness, the struggles and his departure, he calms them again by saying, “Peace I leave you.” … And because he brings up the subject of leaving again, which is enough in itself to trouble them, he again says, “Do not let your heart be troubled, neither let it be afraid.”