In Gen 14:18, Melchizedek, a non-Levite, is called a “priest” using the term כֹהֵ֖ן:
וּמַלְכִּי־צֶ֙דֶק֙ מֶ֣לֶךְ שָׁלֵ֔ם הוֹצִ֖יא לֶ֣חֶם וָיָ֑יִן וְה֥וּא כֹהֵ֖ן לְאֵ֥ל עֶלְיֽוֹן
And King Melchizedek of Salem brought out bread and wine; he was a priest of God Most High. (1985 JPS Tanakh)
Interestingly, Targum Onqelos used a different term, substituting כֹהֵ֖ן for משׁמישׂ (“ministering”). One possibility is that the rabbis sought to avoid, as well as downplay, the use of the title כֹהֵ֖ן for Melchizedek, a non-Levite, notwithstanding the Hebrew Bible using this term for a couple non-Levities (see this article).
For more on this change, see Gudrun Elisabeth Lier, A Redaction History of the Pentateuch Targums: Genesis 1:26-27 in the Exegetical Context of Formative Judaism (Gorgias Dissertations Biblical Studies 53; Piscataway, N.J.: Gorgias Press, 2010), 198-200.
