Christenings and baby dedications are generally done as a substitute for full immersion infant water baptism. The general notion is that because infants have not yet committed sin, and will not be held guilty of sin until after age twelve, they do not yet need to be washed by the blood of Jesus. While this is true, the immersion of infants in the waters of baptism cleanses them not from sins that they have committed, but rather, from the sins of their ancestors.
Every cell in our body, including the reproductive cells, carries in its nucleus a memory of the past. That memory includes every sin that was ever committed by each person in the ancestral lineage. Thus, the infant is born with thousands of years of history recorded in its genes. When that infant is baptized by full immersion, the historical record is erased in regard to sin.
The infant may not be guilty of the sins of its ancestors, but the record of those sins exerts a powerful influence as the child grows and he may unwittingly commit some of the same sins that his ancestors had committed. Baptism by full immersion frees him or her from that compulsion.
Of course, even if the parents choose not to baptize the infant by full immersion, at the very least, the christening or dedication must include giving the infant the indwelling Holy Spirit in order to mark him or her as a legitimate child of God. In fact, the indwelling Holy Spirit can be given to a child even after the christening or dedication ceremony.