Charles Ellicott Commentary Numbers 25:13

"and it shall be unto him, and to his seed after him, the covenant of an everlasting priesthood; because he was jealous for his God, and made atonement for the children of Israel." — Numbers 25:13 (ASV)

And he shall have it, and his seed after him. —The covenant of peace, which was made by the blood of the Cross, and all the blessings which belong to that covenant, stand firm with Christ, and are secured to His spiritual seed. .

Even the covenant of an everlasting priesthood.— Phinehas succeeded his father Eleazar as high priest (Judges 20:28). After a temporary interruption in the succession, which existed in the time of Eli, and continued until the time of David, when it appears there was a joint high-priesthood, the office was restored by Solomon to Zadok, the descendant of Phinehas, and so continued until the gradual dissolution of the Jewish state. Christ’s priesthood is an unchangeable priesthood (Hebrews 7:24): You are a priest for ever after the order of Melchisedec (Hebrews 7:17).