What is to prevent my being baptized?"
Then he ordered the chariot to stop,
and Philip and the eunuch both went down into the water,
and he baptized him.
Incorporated into Christ by Baptism, the person baptized is configured to Christ. Baptism seals the Christian with the indelible spiritual mark (character) of his belonging to Christ. No sin can erase this mark, even if sin prevents Baptism from bearing the fruits of salvation. Given once for all, Baptism cannot be repeated.