From a treatise on the Trinity by St Faustinus
Our Saviour received a bodily anointing and so became a true king and a true priest. Both king and priest he was of his very self; a saviour could be nothing less. Hear in his own words how he himself became a king: I have been appointed king by God on Zion his holy mountain. Hear in the Father’s words that he was a priest: You are a priest for ever in the line of Melchizedek. Aaron was the first under the law to be made a priest by being anointed with chrism, yet the Father does not say, “in the line of Aaron,” lest it be believed that the Saviour’s priesthood could be passed on by inheritance, for at that time Aaron’s priesthood was transmitted by lineal descent. But the Saviour’s priesthood is not inherited because this priest lives on for ever. Therefore Scripture says: You are a priest for ever in the line of Melchizedek.
There is, therefore, a saviour in the flesh who is both a king and a priest, though his anointing was not physical but spiritual. Among the Israelites, those kings and priests who were actually anointed with oil were either kings or priests. No man could be both king and priest; he had to be one or the other. Only Christ was both king and priest; because he had come to fulfil the law, he alone possessed the twofold perfection of kingship and priesthood.
Those who had been anointed with the oil of kingship or priesthood, although they received only one of these anointings, were called messiahs. Our Saviour, however, who is the Christ, was anointed by the Holy Spirit so that the passage in Scripture might be fulfilled: God, your God, has anointed you with the oil of gladness and raised you above your companions. The difference, then, between the one Christ and the many christs is in the anointing, since he was anointed with the oil of gladness, which signifies nothing other than the Holy Spirit.
This we know to be true from the Saviour himself. When he took the book of Isaiah, he opened it and read: The Spirit of the Lord is upon me because he has anointed me. He then said that the prophecy was fulfilled in the hearing of those listening.
Peter, the prince of the apostles, also taught that the chrism which made the Saviour a christ was the Holy Spirit; that is to say, the power of God. When in the Acts of the Apostles Peter spoke to that faithful and merciful man, the centurion, he said among other things: After the baptism which John preached, Jesus of Nazareth, whom God anointed with the Holy Spirit and with power, started out in Galilee and travelled about performing powerful miracles, and freeing all who were possessed by the devil.
So you see that Peter too said that Jesus in his humanity was anointed with the Holy Spirit and with power. Thus Jesus in his humanity truly became the Christ. By the anointing of the Holy Spirit, he was made both king and priest for ever.