Psalm 110:4

4 The Lord has made a strong promise.

He will not change it.

He has promised, ‘You will be a priest for ever,

in the same way that Melchizedek was my priest.’

110:4Melchizedek was both a king and a priest, like Jesus. See Genesis 14:18-20. Later, the Israelite priests had to be Levites, so they were not the right family to be kings. Jesus was not a Levite, but he was a priest like Melchizedek.