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

I baptize you in the name of the Father and of the Son and of the Holy Spirit

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.