Though we might expect Samuel to be the faithful priest, he is explicitly identified as a prophet (and not a priest) in 1 Samuel 3:20. Zadok is later identified as the faithful priest in view.1 The replacement of Abiathar with Zadok is specifically reported as a fulfillment of the word which the Lord spoke at Shiloh to the house of Eli (see 1 Kings 2:26–27, 1 Kings 2:35). Abiathar was descended from Aaron through Ithamar and Eli (1 Chronicles 24:3), whereas as Zadok was descended from Aaron through Eleazar (1 Chronicles 6:3–8; 1 Chronicles 6:50–53).2 According to Numbers 25:10–13, God promised the priesthood to the descendants of Eleazar following the actions of Phinehas. Priests from the house of Eleazar/Zadok would continue to serve in the office of high priest until the rule of Antiochus Epiphanes (175–164 BC), when the priesthood was sold to Menelaus (cf. 1 Chronicles 6:8–15; Ezra 3:2; see 2 Maccabees 4:23–50).3 Ultimately, however, the only truly faithful priest is Jesus Christ. He is designated by God to be the high priest in the order of Melchizedek forever (Hebrews 5:10; Hebrews 6:20).4
35 And I will raise up for myself a faithful priest, who shall do according to what is in my heart and in my mind. And I will build him a sure house, and he shall go in and out before my anointed forever.