John Chrysostom

"And don't grieve," he adds, "the Holy Spirit of God." This is a terrible and startling matter, as he also says in the epistle to the Thessalonians. For there he uses an expression of this sort. "He that rejects, rejects not man, but God." It is the same here. If you utter a reproachful word, if you strike your brother, you are not striking him; rather you are "grieving the Holy Spirit."