We 1st read of Melchizedek in Genesis 14:18, as the 'king of Salem' (which later became known as Jerusalem) and the king of righteousness in the days of Abraham. We find Abrah
…am 'tithing' (a tenth of your riches/wealth/spoils) to God's High Priest, Melchizedek. This Melchizedek was eternal, 'having neither beginning of days, nor end of life...abideth a priest continually' (Hebrews 7:3). This verse indicates that He probably was High Priest even from the time of Adam. The ancient patriarchs financed this ministry of God on Earth by the tithing system (Genesis 48:15), so before the time of Moses, the tithe had been paid directly to Melchizedek. But in Numbers 18: 21, 24, upon Israel's deliverance from Egypt, God sets Aaron's family in highest authority over the Levites. As priests, they were to receive a tithe from the income of all the other Levites. This system continued until Christ's death and resurrection (Hebrews 6:20; 7:25-27). Since Melchizedek was without descent, was 'like unto the Son of God,' and abides as High Priest forever continually, and since Jesus Christ is NOW High Priest, Mechizedek and Jesus Christ are one and the same Person!! Remember what is written in John 1:1-3, both members of the God Family have existed eternally (Psalms 90:2; Hebrews 1:8, 10-12; 1 Timothy 1:17). As impossible for our finite minds to grasp how these two immortal supreme Beings could have always existed, we need to do so. It is like electricity which we still do not understand but we know it exists and is very real. In my Freshman year in a Catholic HS, my teacher/brother taught about electricity using the 'demon theory.' I still don't understand it, but I know it exists. Addendum In the Book of Jasher M. is identified as Shem. ( Full Answer )