So did Jesus receive his divine nature at his baptism? Well, nothing in the four accounts we have or John’s reference to the baptism in 1 John 5 say that.
Moreover, nothing suggests that Jesus became God’s “Son” at that moment. Rather, we’re told that God testified to Jesus’ Sonship both at his baptism and by means of his resurrection. But that’s very different from Jesus’ becoming God’s Son.
For that matter, consider —
(Luk 1:30-33 ESV) 30 And the angel said to her, “Do not be afraid, Mary, for you have found favor with God. 31 And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. 32 He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, 33 and he will reign over the house of Jacob forever, and of his kingdom there will be no end.”
(Luk 2:10-11 ESV) 10 And the angel said to them [the shepherds], “Fear not, for behold, I bring you good news of great joy that will be for all the people. 11 For unto you is born this day in the city of David a Savior, who is Christ [Messiah] the Lord.”
(Luk 2:26-32 ESV) 26 And it had been revealed to him by the Holy Spirit that he would not see death before he had seen the Lord’s Christ [Messiah]. 27 And he came in the Spirit into the temple, and when the parents brought in the child Jesus, to do for him according to the custom of the Law, 28 he took him up in his arms and blessed God and said, 29 “Lord, now you are letting your servant depart in peace, according to your word; 30 for my eyes have seen your salvation 31 that you have prepared in the presence of all peoples, 32 a light for revelation to the Gentiles, and for glory to your people Israel.”
Luke repeatedly records God testifying to Jesus as Messiah and God’s Son at the time of his birth. But he is said to be the Messiah, not that he was going to become the Messiah.
Therefore, I see not a hint that Jesus was merely a man before his baptism. That’s not the point the authors are making. It’s not the conclusion the apostles draw.
On the other hand, it’s entirely possible (but not entirely provable) that the receipt of the Spirit by Jesus at his baptism is the moment he became empowered to do miracles (cf. Luke 4:1, 14). After all, the receipt of the Spirit is associated with miraculous gifts throughout both testaments, and there’s no record of Jesus doing miracles before then (not that we really know).
Moreover, I think it’s a mistake to imagine Jesus as human only before his baptism. After all, what would be the point of the virgin birth if Jesus were just a man during his early life? That theory seems to clearly contradict Luke’s account of Jesus’ conception.
Also, how would the receipt of the Spirit make Jesus a member of the Godhead? Moses had the Spirit. The council of 70 appointed by Moses had the Spirit. Saul had the Spirit. Ezekiel had the Spirit. None were members of the Godhead! Receipt of the Spirit does many wonderful things, but it does not make one God.
Well, then, if Jesus was already God by birth, what did the Spirit accomplish? Well, we don’t exactly know, other than giving testimony to his true nature and place. That we’re told.
It may be that Jesus had so emptied himself (Phil 2:7) by becoming a human infant, that he gave up miraculous abilities until his baptism. After all, he obviously gave up omnipresence, existence as spirit, and who knows what other attributes. He made himself finite. It would make good sense, and fit Philippians 2 very well, to suppose that Jesus had surrendered his miraculous abilities until his baptism. How else could it be true that —
(Heb 4:15 ESV) 15 For we do not have a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sin.
Jesus never used his miraculous abilities for his own benefit. He served others miraculously, but not himself. That was part of his self-emptying (kenosis). Therefore, he did not need those powers until he was ready to enter into his ministry.
And so I take the lack of recorded miracles before his baptism, not as evidence of his mere humanity, but of his deeply Divine nature, because Jesus reveals his divinity most perfectly in his self-emptying — his voluntary surrender of the privileges of heaven in order to suffer along with and on behalf of Israel first, and also for the Gentiles.