Rashi on Numbers 25:13/רש"י על במדבר כ״ה:י״ג

1והיתה לו AND IT SHALL BE UNTO HIM — this covenant of Mine shall be unto him —

2‎ברית 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.

3לאלהיו 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.