Tradition
St. John Chrysostom
Homilies on the Epistle to the Hebrews
Homily 8 › Hebrews 5:6
“As He says also in another place, You are a Priest forever after the order of Melchisedech.” Unto whom now was this spoken?
Who is “after the order of Melchisedech”? No other [than He]. For they all were under the Law, they all kept sabbaths, they all were circumcised; one could not point out any other [than Him].
Source: Homilies on the Epistle to the Hebrews (New Advent)