Isaiah 63 — Three persons of the Trinity in the Old Testament

Isaiah 63:7-10 contains: (1) God the Father — verse 8, "truly they are my people," God acting as father; (2) The Holy Spirit — verse 10, "they rebelled and grieved his Holy Spirit" — combined with Psalm 78:40 (which says Israel "grieved" Yahweh in the wilderness), this demonstrates the Holy Spirit is both personal (can be grieved) and divine; (3) The Angel of His Presence (the "malak") — verse 9 — an allusion to Exodus 23:20 where God sends a messenger before Israel, who bears God's name, and has the divine prerogative to forgive sins (the same authority Jesus exercises in Mark 2). The Angel of the Lord in Zechariah 3 also invokes Yahweh's name against Satan as though a distinct person from Yahweh, consistent with divine plurality throughout the OT.