A faithful priest - This seems to have been spoken of Zadok, who was anointed high priest in the room of Abiathar, the last descendant of the house of Eli; see 1 Kings 2:26, 1 Kings 2:27. Abiathar was removed because he had joined with Adonijah, who had got himself proclaimed king; see 1 Kings 1:7.
I will build him a sure house - I will continue the priesthood in his family.
He shall walk before mine Anointed - He shall minister before Solomon, and the kings which shall reign in the land. The Targum says, "He shall walk משיחי קדם kodam Meshichi, before my Messiah," and the Septuagint expresses it, ενωπιον Χριστου μον, "before my Christ;" for, in their proper and more extended sense, these things are supposed to belong to our great High Priest and the Christian system: but the word may refer to the Israelitish people. See the note on Hebrews 9:26.
Zadok is meant rather than Samuel. The High Priesthood continued in the direct descendants of Zadok as long as the monarchy lasted (see 1 Chronicles 6:8-15).
Mine anointed - in its first sense obviously means the kings of Israel and Judah Psalm 89:20; Zechariah 4:14. But doubtless the use of the term MESSIAH ( Χριστὸς Christos ) here and in 1 Samuel 2:10, is significant, and points to the Lord‘s Christ, in whom the royal and priestly offices are united (Zechariah 6:11-15: see the marginal references). In this connection the substitution of the priesthood after the order of Melchisedec for the Levitical may be foreshadowed under 1 Samuel 2:35 (see Hebrews 7).