Let it be so now, it is proper for us to do this to fulfill all righteousness

Bible tells that John the Baptist born in the line of Aaron's priesthood family - There was in the days of Herod, the king of Judea, a certain priest named Zacharias, of the division of Abijah. His wife was of the daughters of Aaron, and her name was Elizabeth. And they were both righteous before God, walking in all the commandments and ordinances of the Lord blameless. But they had no child, because Elizabeth was barren, and they were both well advanced in years - Luke 1:5-7 But the angel said to him, Do not be afraid, Zacharias, for your prayer is heard; and your wife Elizabeth will bear you a son, and you shall call his name John - Luke 1:13

Aaron's priesthood ministry had three important duties - And when he has made an end of atoning for the Holy Place and the tent of meeting and the altar, he shall present the live goat. And Aaron shall lay both his hands on the head of the live goat, and (1) confess over it all the iniquities of the people of Israel, and all their transgressions, all their sins. And (2) he shall put them on the head of the goat and (3) send it away into the wilderness by the hand of a man who is in readiness. The goat shall bear all their iniquities on itself to a remote area, and he shall let the goat go free in the wilderness - Leviticus 16:20-22

When Jesus Christ began his Ministry, this is how John the Baptist reveled the Lamb of God to the World. 

(1) Aaron should confess over it all the iniquities of the people of Israel, and all their transgressions, all their sins - The next day he (John the Baptist) saw Jesus coming toward him and said, Behold, the Lamb of God, who takes away the sin of the world - John 1:29

(2) Aaron shall put them on the head of the goat - Jesus replied, Let it be so now; it is proper for us to do this to fulfill all righteousness. Then John consented - Matthew 3:15

(3) Send it away into the wilderness by the hand of a man who is in readiness - The Spirit immediately drove him out into the wilderness - Mark 1:12