Baptism is a Christian sacrament of initiation almost invariably with the use of water. It may be performed by sprinkling or pouring water on the head, or by immersing in water either partially or completely, traditionally three times, once for each person of the Trinity. The synoptic gospels recount that John the Baptist baptised Jesus. Baptism is considered a sacrament in most churches, and as an ordinance in others. Baptism according to the Trinitarian formula, which is done in most mainstream Christian denominations, is seen as being a basis for Christian ecumenism, the concept of unity amongst Christians. Baptism is also called christening, although some reserve the word "christening" for the baptism of infants. In certain Christian denominations, such as the Catholic Churches, Eastern Orthodox Churches, Oriental Orthodox Churches, Assyrian Church of the East, and Lutheran Churches, baptism is the door to church membership, with candidates taking baptismal vows. It has also given its name to the Baptist churches and denominations.

A sacrament is a Christian rite that is recognized as being particularly important and significant. There are various views on the existence, number and meaning of such rites. Many Christians consider the sacraments to be a visible symbol of the reality of God, as well as a channel for God's grace. Many denominations, including the Roman Catholic, Lutheran, Presbyterian, Anglican, Methodist, and Reformed, hold to the definition of sacrament formulated by Augustine of Hippo: an outward sign of an inward grace, that has been instituted by Jesus Christ. Sacraments signify God's grace in a way that is outwardly observable to the participant.