CLAIM: Jesus was baptized, but normally, only sinners were baptized by John. Why was Jesus baptized? Was he secretly sinful and needed to repent of this before he could start his ministry?
RESPONSE: Jesus did not need to be baptized for repentance from sin. Instead, he was baptized in order to (1) identify himself with sinful humans and (2) demonstrate his reliance on the Holy Spirit. The author of Hebrews tells us that “[Jesus] had to be made like His brethren in all things” (Heb. 2:17). In Matthew’s account of his baptism, John is shocked that Jesus would be baptized. However, Jesus replied, “Permit it at this time; for in this way it is fitting for us to fulfill all righteousness” (Mt. 3:15). NT scholar William Lane comments, “In submitting to John’s baptism Jesus acknowledges the judgment of God upon Israel. At the same time his baptism signifies that his mission will be to endure the judgment of God.”
Therefore, even at the beginning of his ministry, Jesus was demonstrating that he was becoming identified with sinful humans (c.f. 2 Cor. 5:21). Moreover, by being baptized and visibly receiving the Holy Spirit (Mt. 3:16), Jesus was showing his need to rely on the Holy Spirit in his ministry to perform his miracles and his ministry (Lk. 5:17; Acts 2:22; Acts 10:38). In other words, Jesus’ baptism was a public demonstration of his need to rely on the Spirit—not his own power.
This event was also probably the fulfillment of Jesus being anointed as the King. Jesus’ baptism was a sign that he was being anointed as a king. That is why this passage harkens back to coronation passages like Psalm 2:7 (“You are My Son, today I have begotten You.”) and the chosenness of God’s Messiah Isaiah 42:1 (“Behold, My Servant, whom I uphold; My chosen one in whom My soul delights”).
 Lane, W. L. (1974). The Gospel of Mark (p. 54). Grand Rapids, MI: Wm. B. Eerdmans Publishing Co.