The simple explanation for the practice of infant baptism is the doctrine of original sin, which holds that every human is born "fallen" and with a nature that is corrupt due to the original sin of Adam which was passed on to all his descendants. While most protestants retained this doctrine even after breaking with the Catholic Church, many of them have come to either reject the practice of infant baptism as a means of dealing with original sin. The basic argument is that babies are innocent, as are adults with developmental disabilities and any other person who is unaware or without understanding of their sinful condition. The problem is that there are scriptural passages that support both positions.
While there aren't any instances or examples of infant baptism, the scripture is pretty clear about the condition of mankind and original sin. The following scriptures present this pretty clearly: "All of sinned and come short of the glory of God" (Romans 3:23); "There is none righteous, no not one"(Romans 3:10); "Except a man be born of water and of the spirit he cannot enter into the kingdom of God"(John 3:5);"For as by one man's disobedience many were made sinners, so by the obedience of one shall many made righteous"(Romans 5:19). There is also an array of examples where acts symbolic of redemption through the blood of Christ can be interpreted to function as examples of those who had no choice in the matter being "saved" via the faith of those who had charge over them (The blood on the door posts at the first passover in Exodus 12; Paul's admonition to the Philippian jailer "Believe...and thou shalt be saved, and thy house."; Christ's admonition to allow the children to come to him). However, one has to admit that the Church is taking a great deal on faith (no pun intended) in advocating that spiritual regeneration can be accomplished through infant baptism. Additionally, a more, shall we say, "jaded" scholar has many possibilities (clarification of Church records, a means of generating income, a tool for political purposes) to work with in explaining the institution of infant baptism by the Church, especially since it was added many years after the Church began and has no clear scriptural precedent.
While the practice of infant baptism / christening is not contrary to the scripture, the associated teachings / conclusions that one can achieve salvation by having been christened is very questionable indeed. The scripture treats baptism as a sacrament accompanying salvation, a physical manifestation or declaration of what has taken place spiritually; the scripture clearly dictates that confession of faith in Christ as the Son of God and sacrifice for man's sin is the exclusive means provided by God for human salvation. It is interesting to note that the thief crucified next to Jesus was never baptized at all, let alone as an infant, yet his his faith and confession of that faith was enough for Christ to promise him a place in paradise. Having said that, it is not necessarily contrary to the scriptures to bring infants to Christ via baptism or "dedication" as a means of the parent or guardian to request Christ's protection for their child during the time between birth and when they become responsible for their own state in regard to original sin. On the other hand, it is not contrary to the scripture to allow that a just God would not impute original sin to a child who has yet to come to a knowledge of right and wrong.