וּבְנָיָ֙הוּ֙ בֶּן־יְה֣וֹיָדָ֔ע וְהַכְּרֵתִ֖י וְהַפְּלֵתִ֑י וּבְנֵ֥י דָוִ֖ד כֹּהֲנִ֥ים הָיֽוּ׃ (פ)
Benajahn, Sohn Jojadas, war über die Kreti und die Pleti seine aus fremden Söldlingen zusammengesetzte Leibwache). Die Söhne Davids waren Priester.
Rashi
you are a priest forever because of the speech of Malchizedek From you will emerge the priesthood and the kingship that your children will inherit from Shem your progenitor, the priesthood and the kingship, which were given to him. דִבְרָתִי מלכי-צדק. The “yud” is superfluous, like (Lam. 1:1): “the city that was once so populous (רבתי).” Because of the speech of Malchizedek, because of the command of Malchizedek. You are a priest, Heb. כהן. The word כהן bears the connotation of priesthood and rulership, as (II Sam. 8:18): “and David’s sons were chief officers.”
Rashi
ואתם תהיו לי ממלכת כהנים AND YE SHALL BE UNTO ME A KINGDOM OF כהנים — i. e. princes, just as you say, (II Samuel 8:18) “And the sons of David were princes (כהנים)”, which cannot denote priests since his sons were of the tribe of Judah and not of Levi, the priestly tribe.
Rashbam
ממלכת כהנים, princes; we find this term used in a similar sense when applied to the children of David who were not priests in the hereditary sense such as the sons of Aaron. Compare Samuel II 8,18 ובני דוד כהנים היו, “and the sons of David were ‘priests.’”