Melchizedek Must Be One of the Sons of God
The bible talks about the sons of god visiting the earth in Genesis 6:1, and we do not know what they came to do apart from taking wives of the daughters of men. Melchizedek surely was a priest unto the most high God(one of the sons of God) that has to rule as a king to bless Abram, that is why he is called priest/king.
The bible did not say there were priests of the most high God before him until the time of Aaron. The promise of God was on Abraham as father of nations and God knew he was going to make Israel and give them the order of worship as it is in heaven. More so, God did not bless any man by a priest until Melchizedek the priest/king of Salem blessed Abram.
He knows that a priest must ordain a man to be a priest(even priest ordain kings) and he did it because of the seed of Abram.