Because the Sacrament of Baptism has actual spiritual and salvific effects, certain criteria must be complied with for it to be valid (i.e. , to actually have those effects). Baptism in water is assumed. Violation of some rules regarding baptism render the baptism illicit (i.e. , a violation of the [[canon law|church's laws]], and a [[sin ]] for those who willingly and knowingly participate in it), and yet still valid. For example, if a [[Priestpriest]] introduces some variation in the unauthorized variation in the ceremony, the baptism is still valid so long as certain key criteria are still met, even though the Priest priest has violated the church's law and thus sinned, and so have the other participants if they know the Priestpriest's behaviour is illict.

One of the criteria for validity is that the correct form of words be used. Orthodox use the form "Let this servant of Christ be baptized..." or "This person is baptized by my hands..." Catholics use the form "I baptize you..." However, both churches recognize the other's form as valid. The Catholic church Church teaches that the use of the verb "baptize" is essential.

It is also considered essential that the Trinitarian formula is used. Baptisms from non-Trinitarian churches , such as Oneness PentecostalsPentecostal, are generally not considered valid. There was an ancient controversy over baptism using the formula that Oneness Pentecostals use, with some ancient authorities holding it to be valid. However, this was motivated by the apparent use of that formula at some places in [[scripture]], not by anti-Trinitarian considerations (which might well invalidate the baptism even if that formula is valid). The most significant part, some theologians [[theologian]]s have argued, is not so much the Trinitarian wording, as the Trinitarian intention, and the recognition that the baptism involves all three Persons.

A person, once baptized, cannot be baptized again. There was an ancient practice in some areas of rebaptizing those who had returned to the church from [[heresy]], but that practice has been universally rejected.

 The various jurisdictions of the Orthdox churches Church generally accept baptism performed by other denominations as valid, subject to certain conditions. It is only possible to be baptized once, thus people with valid baptisms from other denominations may not be baptized again on conversion. Instead, for these converts the sacrament of chrismation is performed. However, in some cases it can be difficult to decide if whether the original baptism was in fact valid, so, if there is any doubt, a conditional baptism is employed, in which the officiant says something of the form of "if you are not yet baptized, I baptize you...". The need for conditional baptisms is motivated not only by factual uncertainties regarding the original baptism, but also by the uncertainty of some of the baptismal theology regarding the precise conditions for the validity of baptism . (the The Church holds that one cannot be certain that opinions which are offered by pious theologians, but on which the Church has not made an authoritative pronouncement, are in fact correct, and even authoritative pronouncements can have multiple interpretations which the Church has neither definitively endorsed or rejected.).