Roman Catholic dogma holds that it is a grave sin to baptize a person who has already been baptized. In case of uncertainty about whether a person has been baptized, that person may be baptized conditionally. Such uncertainty may result from questions about whether a church from which someone is converting to Catholicism baptizes in a valid manner. For some Protestant denominations, Catholics do not raise such questions. The Catholic Church has said that the validity of baptisms in Mormon churches and those of some other communions is doubtful. In cases where an emergency baptism is performed, if impure water is used the validity of the baptism may be in question. In that case a conditional baptism is later performed by an ordinary minister of the sacrament with certainly valid matter.
In a typical baptism, the minister of the sacrament (usually a deacon or a priest, but sometimes, especially in when the baptized is in imminent danger of death, a lay person) says
or words to that effect, explicitly naming the three Persons of the Trinity, while pouring or sprinkling water upon the head of the baptized, or immersing them in water. In a conditional baptism, the minister of the sacrament says
If you are not yet baptized, I baptize you in the name of the Father and of the Son and of the Holy Spirit.
From the foregoing it is evident that not all baptism administered by heretics or schismatics is invalid.
The baptism of blood (baptismus sanquinis) is the obtaining of the grace of justification by suffering martyrdom for the faith of Christ.
Moreover, baptism can more readily be applied to infants than the rite of circumcision, and by the ancient law this ceremony had to be deferred till the eighth day after birth, while baptism can be bestowed upon infants immediately after they are born, and in case of necessity even in their mother's womb.