Jesus' baptism was unique in that it was required just as Moses spoke of for Aaron and his sons in Exodus 29:1-4, "And this is the thing that thou shalt do unto them to hallow them, to minister unto me in the priest's office: Take one young bullock, and two rams without blemish, (2) And unleavened bread, and cakes unleavened tempered with oil, and wafers unleavened anointed with oil: of wheaten flour shalt thou make them. (3) And thou shalt put them into one basket, and bring them in the basket, with the bullock and the two rams. (4) And Aaron and his sons thou shalt bring unto the door of the tabernacle of the congregation, and shalt wash them with water." It was required for him as the high priest of the nation. One only need to read the book of Hebrews to see Christ in this role as high priest according to the order of Melchizedek (Hebrews 2:17, 3:1, 4:14-15, 5:5, 5:10, 6:20, 8:1, 9:7, 9:11).
It was the nation of Israel that was to follow in this example "to fulfill all righteousness" as foretold in Exodus 19:6, "And ye shall be unto me a kingdom of priests, and an holy nation. These are the words which thou shalt speak unto the children of Israel." And then God was to use the nation of Israel to reach the Gentiles. We see this in Isaiah 49:6-7, "And he said, It is a light thing that thou shouldest be my servant to raise up the tribes of Jacob, and to restore the preserved of Israel: I will also give thee for a light to the Gentiles, that thou mayest be my salvation unto the end of the earth. (7) Thus saith the LORD, the Redeemer of Israel, and his Holy One, to him whom man despiseth, to him whom the nation abhorreth, to a servant of rulers, Kings shall see and arise, princes also shall worship, because of the LORD that is faithful, and the Holy One of Israel, and he shall choose thee." Also, in Isaiah 60:1-3, "Arise, shine; for thy light is come, and the glory of the LORD is risen upon thee. (2) For, behold, the darkness shall cover the earth, and gross darkness the people: but the LORD shall arise upon thee, and his glory shall be seen upon thee. (3) And the Gentiles shall come to thy light, and kings to the brightness of thy rising." And in Zechariah 8:23, "Thus saith the LORD of hosts; In those days it shall come to pass, that ten men shall take hold out of all languages of the nations, even shall take hold of the skirt of him that is a Jew, saying, We will go with you: for we have heard that God is with you."
In conclusion, Jesus was baptized as the new high priest of Israel according to the order of Melchizedek and they were to follow his example by accepting him as their new Priest and King, he would have then brought in the Kingdom and they would reach the nations. Unfortunatley, that did not happen.