Why do Confirmation?

In fact, the confirmation does not exist in the New Testament, where we find only the baptism. Confirmation is practiced, as well as in the Catholic Church, even in historic Protestant churches which recognize infant baptism as valid. For this, if a person received baptism as a newborn and as an adult he wants to confirm his belonging to the Christian church, must necessarily make confirmation, what else is a rite of public witness confirming the baptism received. If this person does not make the confirmation it means that the baptism wanted for him / her by the parents is not valid.