In the Jewish religion, mikvahs are used symbolically for MANY purposes. Jesus going through John the Baptist's baptism was for His being set apart for His ministry beginning. Of course it wasn't to cleanse Him from sin, nor was it to bring Him "into the sonship of God", since He WAS the Son of God. The Christian ordinance of baptism was not even known about when that event happened.
Adjust your view outside of a pigeonhole idea of a word (i.e., baptism) and you'll be able to understand the concept and purpose of what baptism is really about.
This HERETIC seemed to be a bit, shall we say, dismissing about the TRUE value of baptism:
1 Corinthians 1:15-17
...so that no one would say you were baptized in my name. 16 Now I did baptize also the household of Stephanas; beyond that, I do not know whether I baptized any other. 17 For Christ did not send me to baptize, but to preach the gospel, not in cleverness of speech, so that the cross of Christ would not be made void.
(Poor fool; didn't seem to know whether it was a SIGN or a REQUIREMENT.)
The purpose for Jesus' baptism is stated right in Scripture by John. It was so that John would recognize Jesus and He would be revealed to the world.
John 1:29-34 The next day he saw Jesus coming toward him, and said, Behold, the Lamb of God, who takes away the sin of the world! This is he of whom I said, After me comes a man who ranks before me, because he was before me. I myself did not know him, but for this purpose I came baptizing with water, that he might be revealed to Israel. And John bore witness: I saw the Spirit descend from heaven like a dove, and it remained on him. I myself did not know him, but he who sent me to baptize with water said to me, He on whom you see the Spirit descend and remain, this is he who baptizes with the Holy Spirit. And I have seen and have borne witness that this is the Son of God.
Yes, mikvot historically preceded Christian baptism and are a prefugurement of it. I argued otherwise?