Why was it a big deal for a Jewish person to be baptized?

Answer 1

The fundamental issue was not the physical act of pouring water on the child, but all of the religious and social implications that the baptism would have. While there was some debate among Christians about whether a Non-Christian could be baptized against his will (or the will of his parents if he was underage), there was no debate that the baptized person was irrevocably a Christian and could not be "un-baptized".

The primary problem arises when you realize that it was illegal in all Christian countries for Jewish adults to raise a Christian child. This meant that the child would then be forcibly seized from his parents and given to a Christian family. The reason for this illegality is that it was believed that the Jewish parents would teach the child "untruths" that would condemn the soul of the Christian child to Hell. For this same reason, all contact between the Jewish parents and the child was forbidden. A number of Jewish children who were forcibly baptized were thus ripped from the arms of their parents in order to put them "in good Christian houses". This was a violation of the family's dignity and wholesomeness.

The secondary problem is that Jews believe that their religious views are correct and, therefore, by force of law, their children were taken to be taught incorrect beliefs about the world and not to be taught the correct nature of the world. Jews also value their traditions and it is very important to teach those traditions to the next generation. The loss of this child makes the transmission of these important traditions impossible.

Answer 2

Because becoming a Christian and identifying with them by Baptism was essentially betraying the Judaic faith and changing sides.

Answer 3

The baptism is a Christian tradition, not Jewish. Baptism was introduced in the time of Jesus Christ.