Jesus’ expression “all righteousness” indicates a plurality, and there were indeed a number of reasons his baptism by John fulfilled various aspects of God’s will. We can summarize those reasons by saying that in undergoing baptism, Jesus fulfilled seven significant purposes:
1) He endorsed the baptism of John who was rejected by the religious authorities of that day: “the Pharisees and the experts in the law rejected God's purpose for themselves, because they had not been baptized by John” (Luke 7:30). This was important as John’s ministry fulfilled a number of Old Testament prophecies.
2) He submitted himself to the call and command of a servant of God regarding personal behavior. As part of his overall perfect obedience, Jesus obeyed and taught others to obey what God’s appointed servants decreed: “The teachers of the law and the Pharisees sit in Moses’ seat. So you must be careful to do everything they tell you” (Matthew 23:2-3).
3) He identified directly with the sinful people of Israel, and of the world, for whom he would act as their sin-bearer: “God made him who had no sin to be sin for us” (2 Corinthians 5:21), and the lamb of God who would be sacrificed in a substitutionary death: “the Lamb of God, who takes away the sin of the world” (John 1:29).
4) He fulfilled the ritual requirement of washing placed on all the priests before beginning his own ministry. This was also necessary to serve as a perfect high priest for humanity: “We do have such a high priest … who serves in the sanctuary, the true tabernacle set up by the Lord, not by a mere human being … that is a copy and shadow of what is in heaven” (Hebrews 8:1-5).
5) He showed the important link between baptism and the receipt of the Spirit of God. On the Day of Pentecost, when the Spirit was made freely available, the apostle Peter stressed this same link: “Repent and be baptized … in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit” (Acts 2:38).
6) He provided an opportunity for God to publicly reveal and confirm him as his Son: “This is my Son, whom I love; with him I am well pleased” (Matthew 3:17).
7) He gave an example regarding the practice of baptism for future Christians to follow: “To this you were called, because Christ suffered for you, leaving you an example, that you should follow in his steps”(1 Peter 2:21). This truth lay beneath the commission Jesus gave his disciples directly before his ascension: “go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you” (Matthew 28: 19-20).
There were thus at least seven reasons why Jesus underwent baptism: to support John, to teach us the link between baptism and receipt of the Spirit of God, to display obedience, to serve as an example for us, to provide an opportunity for God to reveal his Son, and not least to be counted as if he were a sinner and thus identified with sinners – while fulfilling the ritual requirements of the law in order to serve as a purified high priest for sinners. Seen in this way, we realize that far from being a ritual footnote to his ministry, Jesus’ baptism was an event of the greatest significance. It was directly after his baptism, of course, that Jesus resisted Satan, called his first disciples, and began to publicly teach, heal the sick, and perform miracles. It is no exaggeration to say that the baptism of Jesus was the necessary preparation for everything that the Son of God would accomplish in his earthly ministry, as well as being the public announcement that his ministry was about to begin.