Heb 7:21. Διὰ τοῦ λέγοντος πρὸς αὐτὸν, by Him that said to Him) On other occasions, he who receives the office swears; here, He who conferred the priesthood swore. There is nothing about this oath in Moses, but in the psalm. See how great authority belongs even to the Psalms! Heb 7:28.-ὤμοσε Κύριος καὶ οὐ μεταμεληθήσεται, the Lord hath sworn and will not repent) So LXX. It is intimated by the oath itself that the decree is one ἀμεταμέλητον, not to be disannulled by any repentance.