Ask three questions and then answer them in reverse order. Question #1: Do you believe the Bible is the Word of God about Himself? Question #2: Should we interpret this ourselves or let the Bible interpret itself? Question #3: What two OT verses inform what’s going on in these texts?
Oneness people believe this is a promise disclosed about the work of the man Jesus. It’s not a conversation between persons of a “trinity.” In another place where the voice came Jesus says explicitly that it was for the people, not for His sake (John 12:30).
Even a growing number of Trinitarian scholars affirm this. V. Phillips Long says, “(At) His baptism Jesus was the true and final prophet, priest and king of God’s appointment (Acts 2:25-32; 3:22-26; Heb. 7).” He further says, “Jesus’ baptism has nothing to do with Him being the 2nd person in the Trinity. He’s being baptized into the Priesthood.” Alister McGrath agrees saying, “Jesus was baptized as an anointing into the priesthood. This is how Jesus manifested himself to Israel.” Many other Trinitarian scholars could be quoted, but the above shows the admission.
So, question #3: What two OT verses are being quoted at the baptism? Psalms 2 is the first which is quoted over OT kings at their coronation. This passage signals a promise at Jesus’ baptism! Jesus as man, a servant, is stepping into His publicly ordained role. Psalms 2 was said over OT Kings when they were ushered into their role, which is the man as King stepping into his office.
The second passage is from Isa. 42 in which Isaiah shows a servant, again a man, whose work will be “well-pleasing” to God. At the baptism, the prophecy of the “well-beloved obedient Son” is here entering His role, as man. The OT passages link the man Jesus’ ministry to the prophecies of Isaiah. They say God would lay “the iniquity of us all” (Isa. 53:6) upon one man, who would be stricken “for the transgression of my people” (Isa. 53:8). Isaiah 42 anticipates not only His call to suffer but also is a public declaration – this suffering would lead to glory!
Question #2: Should we interpret the NT quotation ourselves or let the Bible interpret itself? It’s not about a person of the Trinity; even OT scholars that are Trinitarian admit this (see previous quotes). What is the OT promise? An eschatological one. This man is the Christ! His baptism proclaimed a new age (like Noah and the dove) where the Spirit will be available to every person (thus Joel 2 and Acts 2). He’s the only access!
Of course, the last question follows. Do you believe the Bible is the Word of God about Himself? Then you must let the OT show this isn’t about persons in a trinity but the prophecy regarding what God would do as the man Christ Jesus. That’s why Scripture states of this man, “Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.”