Psalms 110:4

“The Lord hath sworn - He has confirmed the appointment of the Messiah by a solemn oath, or as by an oath. That is, It is as sure and fixed as if he had taken an oath. Compare Hebrews 6:13. The “time,” so to speak, if the word time can be applied to transactions in a past eternity, was that when he was designated in the divine purpose as Messiah; in the eternal counsels of God. Compare Psalms 2:7. And will not repent - Will not change his purpose. Thou art a priest forever after the order of Melchizedek - The word rendered “order” here means properly a word, a thing, a matter; hence, a way or manner. The meaning here is, that he would be a priest “after the manner” of Melchizedek; or, such a…”

“Sworn - That this priesthood might be made sure and irrevocable. Psalms 110:5”

“110:4 The Lord had made an unbreakable oath and covenant, so his apparent rejection of David’s dynasty (89:38) in the Exile had led to a crisis. The placement of Ps 110 in Book Five reaffirms the future of the dynasty: David’s descendant must conform to God’s standard of integrity (Ps 101). His ministry will be transformed as he serves under God’s kingship (Pss 93–100). • You are a priest forever: The role of God’s messianic ruler changes. The priestly kingship of David and his descendants extended to serving as patrons of the Temple. David had the Ark brought to Jerusalem (2 Sam 6:15) and arranged for the Temple service (1 Chr 6:31-48; 15:11-26; 16:4-42; 23:4-32; 25:1). Solomon supervised…”

“Lord: Psa 89:34-36; Heb 5:6, Heb 6:13-18, Heb 7:28 will not: Num 23:19 Thou: Gen 14:18; Zac 6:13; Heb 6:20, Heb 7:1-3, Heb 7:11, Heb 7:17; Rev 1:6 Psalms 110:5”