In the Old Testament, Abraham received a blessing from Melchizedek, the priest of God. (Genesis 14:17-20)

Genesis 14:17 And the king of Sodom went out to meet him after his return from the slaughter of Chedorlaomer, and of the kings that were with him, at the valley of Shaveh, which is the king's dale. 18 And Melchizedek king of Salem brought forth bread and wine: and he was the priest of the most high God. 19 And he blessed him, and said, Blessed be Abram of the most high God, possessor of heaven and earth: 20 And blessed be the most high God, which hath delivered thine enemies into thy hand. And he gave him tithes of all.

In the Old Testament, God said that the coming Christ was an eternal priest according to the order of Melchizedek. (Psalm 110:4)

Psalm 110:4 The Lord hath sworn, and will not repent, Thou art a priest for ever after the order of Melchizedek.

Jesus, the Son of God, is the priest of God according to the order of Melchizedek. (Heb 7:1-3, 11,17,21, Heb 5:6,8-10)

Heb 7:1 For this Melchisedec, king of Salem, priest of the most high God, who met Abraham returning from the slaughter of the kings, and blessed him; 2 To whom also Abraham gave a tenth part of all; first being by interpretation King of righteousness, and after that also King of Salem, which is, King of peace; 3 Without father, without mother, without descent, having neither beginning of days, nor end of life; but made like unto the Son of God; abideth a priest continually. 11 If therefore perfection were by the Levitical priesthood, (for under it the people received the law,) what further need was there that another priest should rise after the order of Melchisedec, and not be called after the order of Aaron? 17 For he testifieth, Thou art a priest for ever after the order of Melchisedec. 21 For he testifieth, Thou art a priest for ever after the order of Melchisedec.

Heb 5:6 As he saith also in another place, Thou art a priest for ever after the order of Melchisedec. 8 Though he were a Son, yet learned he obedience by the things which he suffered; 9 And being made perfect, he became the author of eternal salvation unto all them that obey him; 10 Called of God an high priest after the order of Melchisedec.