바미드바르 (민수기) 25:13

והיתה לו AND IT SHALL BE UNTO HIM — this covenant of Mine shall be unto him — ‎ברית כהנת עולם A COVENANT OF AN EVERLASTING PRIESTHOOD — for although the priesthood had already been given to Aaron’s descendants (cf. Exodus 28:40-41), it was given only to Aaron and his sons who had been anointed together with him and to their offspring whom they might beget after they had been anointed. But Phineas who had been born prior to that and had not been anointed, had not as yet attained the status of priesthood until now. So, too, do we read in Zevachim 101b: Phineas did not become a priest until he had slain Zimri. לאלהיו means [HE WAS ZEALOUS] FOR THE SAKE OF HIS GOD, just as, (Numbers 11:29) “Are you zealous for My sake (לי)?” and (Zechariah 8:2) “I am zealous לציון", i.e., for Zion’s sake.

וְהָ֤יְתָה לּוֹ֙ וּלְזַרְע֣וֹ אַחֲרָ֔יו בְּרִ֖ית כְּהֻנַּ֣ת עוֹלָ֑ם תַּ֗חַת אֲשֶׁ֤ר קִנֵּא֙ לֵֽאלֹהָ֔יו וַיְכַפֵּ֖ר עַל־בְּנֵ֥י יִשְׂרָאֵֽל׃

and he shall have it, and his seed after him, the covenant of an everlasting priesthood; because he was zealous for his God, and made atonement for the children of Yisra᾽el.