Hebrews 7:6 (KJV)

For this Melchisedec, king of Salem, priest of the most high God, who met Abraham returning from the slaughter of the kings, and blessed him;

To whom also Abraham gave a tenth part of all; first being by interpretation King of righteousness, and after that also King of Salem, which is, King of peace;

Without father, without mother, without descent, having neither beginning of days, nor end of life; but made like unto the Son of God; abideth a priest continually.

Now consider how great this man was, unto whom even the patriarch Abraham gave the tenth of the spoils.

And verily they that are of the sons of Levi, who receive the office of the priesthood, have a commandment to take tithes of the people according to the law, that is, of their brethren, though they come out of the loins of Abraham:

And without all contradiction the less is blessed of the better.

And here men that die receive tithes; but there he receiveth them, of whom it is witnessed that he liveth.

And as I may so say, Levi also, who receiveth tithes, payed tithes in Abraham.

For he was yet in the loins of his father, when Melchisedec met him.

If therefore perfection were by the Levitical priesthood, (for under it the people received the law,) what further need was there that another priest should rise after the order of Melchisedec, and not be called after the order of Aaron?