Question: If the priesthood was for the tribe of Levi only, then how could Jesus be considered High Priest?
Answer:Well, of course, Jesus was from the tribe of Judah as to his human nature, and you’re right that Israel’s high priest was supposed to be a Levite from Aaron’s family. But the author of Hebrews asserts that Jesus was the legitimate high priest after the order of Melchizedek. He bases this on a prophecy found in Psalm 110:4 by David that addresses David’s Lord as “a priest forever, in the order of Melchizedek” and argues that this is a superior priesthood because Levi paid tithes, so to speak, through Abraham to Melchizedek (Hebrews 7). He also bases its superiority on the fact that, unlike the priests of Aaron’s descent, Jesus has an indestructible life, as did Melchizedek figuratively since no origin or ending is ever mentioned as to his life. All of this is instructively announced and explained in Hebrews 7.
Does God have the right to institute a new priesthood outside the bounds of Aaron’s? Of course! Aaron’s priesthood was a type of the Messiah to come, but so was Melchizedek’s, and Jesus fulfills the type perfectly in a way neither of them could do themselves.