The Lord has sworn and will not change His mind, “You are a priest forever, after the order of Melchizedek.”

Psalm 110:4

In Genesis 14: 18-20, we read about the mysterious figure of Melchizedek,
And Melchizedek, king of Salem, brought out bread and wine; he was priest of God Most High, and he blessed him and said, “Blessed be Abram by God Most High, maker of heaven and earth; and blessed be God Most High, who has delivered your enemies into your hand!” And Abraham gave him a tenth of everything.
Melchizedek makes a quick appearance in Genesis. He is a king. He is also a priest. He brought out bread and wine, prefiguring the Eucharist. Melchizedek blessed Abraham (the Patriarch of Judaism) and Abraham gave Melchizedek a tenth of everything he had. Abraham recognized Melchizedek as one greater than he.
The name “Melchizedek” means “king of righteousness” (Hebrews 7:2) and the king of Salem means “king of peace.” (7:2) Both of these titles prefigure the identity, titles, and work of Christ. Melchizedek is presented without genealogy—He is descended from no one. He is without earthly origin.
The Psalms in many instances foretell or prefigure the coming of the Messiah. Psalm 110 is an example of this. This Psalm prefigures the victory of Christ, which personifies the Old Testament figure of Melchizedek.
The promise that the Lord has sworn in verse 4 is the promise to redeem Israel. The “YOU are a priest forever, after the order of Melchizedek” is an acknowledgement that the Christ is the fulfillment of that promise.
Psalm 110:3-4 is chanted at the Small Entrance on the Feast of the Nativity, reminding us all that Christ is the One Whom the Lord swore He would send, a promise that He will not retract. The essence of the Nativity is not the baby in the manger, but the promise that God has fulfilled, to send His Only-Begotten Son, the uncreated Logos, to become part of creation.
I always read Psalm 110 in a very personal way. After all, I am, by God’s grace and despite my unworthiness, one of God’s priests. I am reminded in this Psalm that the priesthood is forever. It is not something that is turned on and shut off at will. It is a part of who I am, not just what I do.
By extension, everyone who is baptized enters into the “Royal Priesthood” (But you are a chosen race, a royal priesthood, a holy nation, God’s own people, that you may declare the wonderful deeds of Him Who called you out of darkness into His marvelous light. I Peter 2:9). This is not the ordained priesthood. This does not mean that every Christian can take on the role of the priest. Nor does it discount the need for an ordained priesthood. What it means is that like the priesthood of Melchizedek, which has no origin, everyone who is one of God’s children, can now trace their ancestry to the uncreated God, that our life is a gift from God. And just as Psalm 110:4 reminds the ordained priests that they are priests at all times, so it should also remind the laity, any other Christian person, that our Christianity is not something we do—that we turn it on and turn it off as is convenient—but it is part of our identity. It is something that we are, at all times and in all places. We are Christians forever, and at all times.
The other comfort from this Psalm also comes from verse 4, that “the Lord has sworn and will not change his mind.” God makes good on His promises. He promised a Messiah to His people. He didn’t tell them exactly when He would come. They didn’t all recognize Him when He came, or understand exactly His purpose. But God made good on His promise. Christ came. Christ died on the cross. Christ was resurrected from the dead. And Christ has opened the path to paradise. Today, we live in the expectation of going to Paradise. God has sworn this promise to us. He will not change His mind. Those who believe and who live out their faith by loving and serving others in God’s name will receive God’s grace and be allowed to enter into His Heavenly Kingdom.
Our response is to personify His Royal Priesthood forever, meaning today and at all times.
The Lords says to my lord: “Sit at my right hand, till I make your enemies your footstool.” The Lord sends forth from Zion your mighty scepter. Rule in the midst of your foes! Your people will offer themselves freely on the day you lead your host upon the holy mountains. From the womb of the morning like the dew your youth will come to you. The Lord has sworn and will not change His mind, “You are a priest forever, after the order of Melchizedek.” The Lord is at your right hand; He will shatter kings on the day of His wrath. He will execute judgment among the nations, filling them with corpses; He will shatter chiefs over the wide earth. He will drink from the brook by the way; therefore, He will lift up His head. Psalm 110
Each of us will do many things today. BE a Christian while doing what you do today. Because being a Christian isn’t something we do. It’s something we ARE, at all times and in all places.