It is very common in most Christian denominations to regard Baptism as a New Testament replacement for circumcision. And since Israelite children were to be circumcised on the eighth day, this becomes justification for infant baptism.
First of all, the problem as well as the sprinkling vs. immersion issue wouldn't exist if we made it a point to understand baptizo in its original Jewish cultural context: That of the mikveh. A mikveh was a ritual immersion which symbolized cleansing from sin and ritual impurity. When a Gentile became circumcised, they also immersed in the mikveh not only to be ritually purified, but to symbolize dying (being "buried" in the water) to their old, heathen lives and being reborn from the water as from the womb's amniotic fluid to their new lives as Jews.
Yochanan the Immerser, aka John the Baptist, was not creating a new ritual, but was using it in a new way: Instead of only Gentiles "dying" to their old lives and being raised anew as Jews, Jews who had been living in sin were "dying" and being "reborn" in an act of repentence. However, that wouldn't necessarily be the last time they ever ritually immersed. They would immerse if they became ritually unclean, for example, before going to the Temple. Jews also commonly (and still do today) ritually immerse on Yom Kippur as a demonstration of repentence.
It's truly sad that we have so far removed the Gospel from its original context that we get into debates over whether a person who was "baptized" as a child needs to be baptized upon coming into an adult faith. Indeed, during the Reformation wars, the followers of Zwingli were known for drowing the Anabaptists (so called because they insisted on the necessity of being "baptized again" upon receiving Christ).
However, while I believe that ritually immersing need not be restricted to the baptism one receives upon receiving Yeshua in faith, let's concentrate on that particular mikveh for a moment, and call it "baptism" for the sake of a convenient modern term. The question is, does baptism replace circumcision, and should infants therefore be circumcised?
The Biblical answer is clearly no. One has to reject at least one of two clear Biblical teachings in order to hold to infant baptism:
1) That we are not born into the Messiah's Covenant (as one is into the Abrahamic and Mosaic Covenants), but are re-born into it (Jn. 3).As we've seen in detail, no covenant in Scripture aborogates a previous one, as we've seen in previous entries, nor does the New Covenant replace the Torah. Indeed, Yeshua affirmed the whole Torah, never once criticized the Torah in any way, and only challenged the man-made traditions which either turned it into a burden or which perverted or aborogated it.
2) Those who are immersed (baptized) are identifying themselves with the Messiah's death--i.e., dying to self--and are being raised again with Him into a new life in which one is dedicated to God (Col. 2:10-12, Gal. 3:27). An infant hasn't got an "old life" to die to yet.
So then, for Messianic Jewish parents to circumcise their child as a mark that he was born into the Abrahamic and Mosaic Covenants is not only appropriate, but commanded in Scripture. A Jewish child is the natural inheritor of those covenants from birth, while the Christian is only adopted into Abraham's greater family at the time of their Spiritual circumcision, when they personally enter into a covenant with the Messiah. Am I then claiming that a Jew is saved by virtue of being a Jew? Not at all! For those who are born into the Mosaic Covenant are likewise subject to the curses pronounced by the Torah for those who disobey it, and there is no one who keeps the Torah perfectly, no one who is without sin! It was these curses that Yeshua took on Himself at the Cross so that we would not have to suffer them, and the Jew needs that redemption most of all, for they are held to the highest standard of all by the God of Abraham, Isaac, and Jacob.
This is why a Jew is both circumcised at birth and receives mikveh at the time of his Spiritual Rebirth--they are the seals of two separate covenants.
For Gentile Christian parents to dedicate their child to the Lord before the congregation is likewise appropriate. Such a dedication declares to all that they will raise the child to love and fear God and follow His commandments, but does not pretend to replace the child's personal re-birth.
What is not appropriate is to "water down" (if you'll pardon the pun) the immersion into the Messiah by sprinkling those who have not yet been born again in the Spirit and have not received the Spirit's circumcision of the heart (the promise of Jeremiah's prophecy) and calling it "baptism."
To summarize: Jews are born, and that is why they are circumcised immediately after birth. However, believers in the Messiah Yeshua, whether we call them Christians or Messianics, are not born but re-born, so to "circumcise" them with infant baptism, years before they are born in the Spirit, would be like trying to circumcise a Jewish child in the womb. The only way one can justify baptizing infants is to claim that they need not be born again in the Spirit to enter the Messiah's Covenant (which would be a complete repudiation of the entire NT) or to claim that even those who are truly "born again" may fall away from the Covenant.