“In the time of Herod king of Judea there was a priest named Zechariah, who belonged to the priestly division of Abijah; his wife Elizabeth was also a descendant of Aaron.” Luke 1:5, NIV.
Here we have a pretty obscure verse that shows us something very profound. John the Baptist is descended from the Levitical priesthood, and thus represents the Law. Both his mother and father were full blooded descendants of Aaron. Aaron was selected as the first priest, so saying they are descended from Aaron is saying they are high-pedigree priests:
““Then bring near to yourself Aaron your brother, and his sons with him, from among the sons of Israel, to minister as priest to Me--Aaron, Nadab and Abihu, Eleazar and Ithamar, Aaron’s sons.” Exodus 28:1, NASB.
They are known as Levites because Aaron was descended from one of Jacob's sons, Levi:
“Then the anger of the LORD burned against Moses, and He said, “Is there not your brother Aaron the Levite? I know that he speaks fluently. And moreover, behold, he is coming out to meet you; when he sees you, he will be glad in his heart.” Exodus 4:14, NASB.
““And I have given the Levites as a gift to Aaron and to his sons from among the sons of Israel, to perform the service of the sons of Israel at the tent of meeting, and to make atonement on behalf of the sons of Israel, that there may be no plague among the sons of Israel by their coming near to the sanctuary.”” Numbers 8:19, NASB.
However, Jesus is not descended from the line of Aaron, but from the line of Judah.
So, when John baptizes Jesus, we have Jesus submitting to and fulfilling the law. John baptizes as one who represents and is under the law, for repentance from sins. As we look through John's ministry we see that it has to do with declaring the need for repentance, thus representing the law.
However, John himself says that he is subservient to Jesus:
““a voice of one calling in the desert, ‘Prepare the way for the Lord, make straight paths for him.’” And so John came, baptising in the desert region and preaching a baptism of repentance for the forgiveness of sins. The whole Judean countryside and all the people of Jerusalem went out to him. Confessing their sins, they were baptised by him in the Jordan River. John wore clothing made of camel’s hair, with a leather belt round his waist, and he ate locusts and wild honey. And this was his message: “After me will come one more powerful than I, the thongs of whose sandals I am not worthy to stoop down and untie.” Mark 1:3-7, NIV.
“John answered and said, “A man can receive nothing, unless it has been given him from heaven. “You yourselves bear me witness, that I said, ‘I am not the Christ,’ but, ‘I have been sent before Him.’ “He who has the bride is the bridegroom; but the friend of the bridegroom, who stands and hears him, rejoices greatly because of the bridegroom’s voice. And so this joy of mine has been made full. “He must increase, but I must decrease. “He who comes from above is above all, he who is of the earth is from the earth and speaks of the earth. He who comes from heaven is above all. “What He has seen and heard, of that He bears witness; and no man receives His witness. “He who has received His witness has set his seal to this, that God is true. “For He whom God has sent speaks the words of God; for He gives the Spirit without measure.” John 3:27-34, NASB.
So we have the one who represents the law, preparing and making the way, and pointing to Christ, saying publicly that he is less than Christ. This is a picture of the law acting as a tutor to lead to God's work of grace, and of the law being less than grace.
“John *bore witness of Him, and cried out, saying, “This was He of whom I said, ‘He who comes after me has a higher rank than I, for He existed before me.’” For of His fulness we have all received, and grace upon grace. For the Law was given through Moses; grace and truth were realized through Jesus Christ. No man has seen God at any time; the only begotten God, who is in the bosom of the Father, He has explained Him.” John 1:15-18, NASB.
What this says to me is that the idea of law being an agent which cannot produce righteousness but which leads one to grace, is a message that God spoke even in the grand sweep of long Jewish lineage over thousands of years. The message of grace is His message, His primary message, a message He planned from the very beginning, and which He is using the Jewish nation and the grand sweep of history to teach us.