God saddled/blessed Paul with two uphill battles. Number one, Paul had to explain to Peter and the eleven how he could teach justification by faith apart from works of law to the nations. Number two, it fell upon Paul to explain to Israel in general that, even though Jesus Christ was out of Judah and not Levi, he could still be the Chief Priest in the Millennium.

These were both extremely hard sells.  

God—and Scriptural precedent—to the rescue. To explain justification by faith among the nations to the Lord’s disciples, God inspired Paul to recall the Genesis 15 account of Abraham being reckoned righteous due simply to believing God. To explain to Israel-at-large the viability of Christ as Chief Priest, God points Paul to the Genesis 14 account of Abraham meeting with the great and mysterious Melchizedek, a king and priest of the Most High God before Israel (and thus Levi) even existed. and tithing him, that his, recognizing him as superior. Not only this, but Melchizedek blesses Abraham­—nothing remarkable there except for the fact (and the Jews well knew this) that the inferior is blessed by the greater.

Oh, and Psalm 110: 4 helped a WHOLE lot.