Roman Catholicism has repeatedly throughout history declared a mortal sin "rebaptizing". The Donatists and Novatians were killed for it in the 4th-6th centuries A.D. The Anabaptists were killed for it during the Reformation ('ana' meaning 'again' or 'rebaptists'). Roman Catholicism baptizes you as an infant and to get baptized again has been a crime Catholicism would punish with death.
Ironically, Catholicism is once again on the opposite side from the Apostle whom it allegedly most respects, Peter. Peter said that we are not saved by any physical action in baptism, but by the answer of a good conscience toward God:
- 1 Peter 3:20-21 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water. The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ:
It could also be pointed out that Biblical baptism was always of those old enough to follow Christ, and at one point, Philip the Apostle refused to baptize the treasurer of Ethiopia unless he first admitted to believing with all his heart:
- Acts 8:37-38 And Philip said, If thou believest with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God. And he commanded the chariot to stand still: and they went down both into the water, both Philip and the eunuch; and he baptized him.
Obviously this is not something one considers a baby capable of admitting to or doing. This is in keeping with what Jesus said, that one must not only be baptized but believe also in order to be saved:
- Mark 16:16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
Philip the Apostle would not have baptized infants, yet Roman Catholicism does. Clearly they have begun physical ordinances and commandments of men, the same thing the Pharisees once did, teaching traditions that made the commandments of God of no effect:
- Matthew 15:1-9 Then came to Jesus scribes and Pharisees, which were of Jerusalem, saying, Why do thy disciples transgress the tradition of the elders? for they wash not their hands when they eat bread. But he answered and said unto them, Why do ye also transgress the commandment of God by your tradition? For God commanded, saying, Honour thy father and mother: and, He that curseth father or mother, let him die the death. But ye say, Whosoever shall say to his father or his mother, It is a gift, by whatsoever thou mightest be profited by me; And honour not his father or his mother, he shall be free. Thus have ye made the commandment of God of none effect by your tradition. Ye hypocrites, well did Esaias prophesy of you, saying, This people draweth nigh unto me with their mouth, and honoureth me with their lips; but their heart is far from me. But in vain they do worship me, teaching for doctrines the commandments of men.
Roman Catholicism is in effect a combination of the two forces that killed Jesus, the Roman leaders with their warfaring paganism, and the Jewish leaders, the Pharisees, with their religious orthodoxy, their traditions and doctrines. It is the worst of both worlds, so to speak.