The Melchezedek order pre-exists the Levitical order
4 Just think how great he was: Even the patriarch Abraham gave him a tenth of the plunder!5 Now the law requires the descendants of Levi who become priests to collect a tenth from the people—that is, from their fellow Israelites—even though they also are descended from Abraham.6 This man, however, did not trace his descent from Levi, yet he collected a tenth from Abraham and blessed him who had the promises.7 And without doubt the lesser is blessed by the greater.
Those after the order of Melchezedek were what we now call Christians.