Affusion (la. affusio) is a method of baptism where water is poured on the head of the person being baptized. The word "affusion" comes from the Latin affusio, meaning "to pour on". Affusion is one of four methods of baptism used by Christians, which also include total submersion baptism, partial immersion baptism, and sprinkling. Christian denominations that baptize by affusion do not deny the legitimacy of baptizing by immersion or aspersion; rather, they consider that affusion is a sufficient, if not preferable, method of baptism.
Affusion and aspersion tend to be practiced by Christian denominations that also practice infant baptism. This may be due to the practical difficulties of totally immersing an infant underwater. However, Eastern Orthodox and some Roman Catholics practice infant immersion. Amish, Old Order Mennonites, and Conservative Mennonites still practice baptism by pouring.
History of affusion 
Affusion became the standard practice in the western church around the 10th century, but was in use much earlier. The earliest explicit reference to baptism by affusion occurs in the Didache (c. AD 100), the seventh chapter of which gives instructions on how to baptize, which include affusion:
…But if you have no living water, baptize into other water; and if you cannot do so in cold water, do so in warm. But if you have neither, pour out water three times upon the head into the name of Father and Son and Holy Spirit (emphasis added)
This text implies that early Christians saw affusion is a viable alternative to immersion when no living water (i.e. running water like a river or spring) or cold water is available.
Acts of various martyrs show that many were baptized in prison, while awaiting martyrdom; immersion would have been impossible. The most common use, however, was for ill or dying people who could not rise from their beds. It was consequently known as "baptism of the sick." Receiving this baptism was regarded as a bar to Holy Orders, but this sprang from the person's having put off baptism until the last moment—a practice that in the fourth century became common, with people enrolling as catechumens but not being baptized for years or decades. While the practice was decried at the time, the intent of the criticism was not to encourage baptism by immersion, but to refrain from delaying baptism.
Affusion and the Bible 
In the New Testament book of Acts, the baptism of the Holy Spirit is sometimes described, as a “pouring out” of the Holy Spirit (Acts 2:17,18,33; Acts 10:45). Luke, the possible author of Acts, uses the word "baptism" to describe a "pouring," which seems to indicate that the word "baptism" can refer to pouring and not just dipping or immersing. It may also indicate that Luke’s concept of baptism includes, or allows for, baptism by pouring. For instance, on Pentecost, the disciples were baptized with the Holy Spirit by having the Spirit “poured out” on them from heaven not by being dipped in the Holy Spirit until they were completely immersed.
Submersionists say that passages like these do not directly speak to the issue of water baptism because they are, strictly speaking, about baptism with the Holy Spirit. Affusionists think they indirectly apply to water baptism, though, by telling us something about the general concept of baptism, regardless of whether the medium of baptism is water or Spirit.
Affusionists see more evidence that Luke’s concept of baptism includes pouring by noting an additional possible connection between water baptism and Holy Spirit baptism. In Acts 10, Peter is “astonished that the gift of the Holy Spirit had been poured out even on the Gentiles” (Acts 10:45)). Peter responds by saying, “Can anyone keep these people from being baptized with water? They have received the Holy Spirit just as we have" (Acts 10:47)).
Affusionists read Peter to be saying "by having the Spirit poured out on them, these people already have been baptized with the Spirit, so why not actually baptize them with water." They understand Peter’s words to imply that water baptism is a symbolic picture of the Holy Spirit baptism. If this is right, affusionists contend, then water baptism should be, or, at least, can be, by pouring, because the baptism with the Holy Spirit of which it is a picture occurs by pouring.
See also 
- Believers Baptism
- Baptism of desire
- Baptism of the Holy Spirit
- Baptism of Jesus
- Conditional baptism
- John the Baptist
- Infant baptism
- "Affusion" Dictionary.com
- "There are four principal methods of baptizing: (1) submersion (or total immersion) ; (2) immersion, when the head is dipped with or without the candidate standing in the water; (3) affusion, when water is poured over the head and (4) …" (John Gordon Davies, The Architectural Setting of Baptism. Barrie and Rockliff 1962. p. 23).
- "The four principal modes are: 1. Submersion; or total immersion, where the candidate goes briefly but entirely below the water, on the model of those baptised by John in the River Jordan; 2. Immersion; where the head, as the prime seat of Man's rational and spiritual being, is in some way submerged, with or without the candidate having to stand in the same container of water; 3. Affusion; …" (Charles Thomas, Christianity in Roman Britain to A.D. 500. University of California Press 1981 ISBN 0-520-04392-8. p, 204
- "In the early Church baptism was by one of four methods: complete submersion of the entire body, immersion of the head, affusion or pouring, or aspersion or sprinkling" (Steven J. Schloeder, Architecture in Communion. Ignatius Press 1998 ISBN 0-89870-631-9. p. 113).
- "There have been four different modes of conferring baptism throughout history: (1) submersion, also called dipping, in which the candidate is completely submerged under the baptismal waters; (2) immersion, in which the candidate stands or kneels in rather shallow water, and the water is either poured over the head of the candidate or the candidate's head itself is pushed partially into the water; (3) affusion, in which water is poured over the head of the candidate; and (4) aspersion …" (S. Anita Stauffer, On Baptismal Fonts: Ancient and Modern. Grove Books 1994, pp. 9-10).
- "Baptistery", Encyclopædia Britannica, 2007. Retrieved March 7, 2007.
- "Didache" in The Ante-Nicene Fathers: Translations of the Writings of the Fathers down to A.D. 325. vol. 1. Roberts, Alexander; Donaldson, James; and Coxe, A. Cleveland, Eds. (Grand Rapids: Wm. B. Eerdmans Publishing Company), 1951 (original pub. in U.S. 1885).