This is an aside, but being sinless is not tantamount to being perfect. Clearly, according to the Scriptures, Jesus was sinless throughout His life. Notwithstanding, he had to learn obedience and had to bemade perfect.We know that Christ was without sin throughout His life on earth (or possibly beyond an age of accountability so that He was without taint of any guilt from sin). I have assumed that this was because from birth the fullness of the Spirit dwelt with Him completely all His life and that we could do likewise in measure to the fullness of the Spirit in our own lives.
Although he was a son, he learned obedience through what he suffered. And being made perfect, he became the source of eternal salvation to all who obey him ... (Hebrews 5:8,9 ESV)
While Jesus was on earth, his Spirit was confined to his body — and Jesus IS the Spirit (2Co 3:17,18). The Spirit of the Father (not a third Person) came upon him at his baptism. And yet that Spirit is of the same essence as his own, since he and the Father share the same Spirit — the holy Spirit.
Jesus breathed on His disciples and said, "Receive the Holy Spirit" (John 20:22). I think Jesus imparted his own Spirit to them at that time. The Spirit of Jesus was with them at that time, but when the Spirit fell on them on that special day of Pentecost, it dwelled within them and endued them with power. Perhaps it was a similar case with Jesus Himself. Clearly the Spirit of God the Father was WITH Him long before his baptism, for he communed with His Father constantly, and knew the Father's will, and did it. But perhaps at the time of his baptism, the Spirit of the Father dwelled within him in a permanent and fuller way so that He was given more spiritual power as a result.I understand we cannot enjoy the fullness of the Spirit unless and until we have been justified and transformed by our belief in Christ and our commitment to Him as Lord as evidenced in our baptism. I also understand that this wasn't even possible before Christ's sacrifice for anyone who was naturally conceived. But, even as to Christ, it seems that this was a special instance that didn't occur for whatever reason prior to His baptism. So, why not?
Perhaps I've already answered this question in the previous paragraph.And why did the Spirit need to come upon Him at His baptism? To what end if He was already fully in communion with the Father through His own Spirit?