Eucharistic theology treats doctrines of the Holy Eucharist. It exists exclusively in Christian and quasi-Christian religions, as others generally do not contain a Eucharistic ceremony. An exception would be the theistic Satanists, some of whom recognise transubstantiation and consequently gather in Black Masses to desecrate the Sacrament, a practice which has decreased in frequency since the advent of neo-Satanic faiths.

Orthodox Christianity

Catholicism

Catholic reverence for the Eucharist is quite fervent and the doctrines of the Church thereof are immutable, despite common dissent from them. In the Catholic Church, the Holy Eucharist is taught to be the Body, Blood, Soul, and Divinity of Jesus Christ (this doctrine is referred to as transubstantiation). The actual transformation of bread and wine occurs at the priest's words, "This is My Body" and "This is My Blood." At that point, the accidents of bread and wine remain, i.e., it would appear to all senses that these continue to exist, while the substance has been entirely altered, a position succintly summarised by St. Thomas Aquinas's hymn, "Adoro Te Devote" (). Consequently, for fear of desecration, the Eucharist may not be received by any in a state of mortal sin, nor (generally) by non-Catholics. However, it is permitted on extremely rare occasions that a Protestant or schismatic acknowledging the teachings of the Church on

Additionally, the Eucharist enables perpetuation of Christ's Sacrifice on Golgotha, which is the intent of the Mass. Despite anti-Catholic claims to the contrary, this perpetuation does not imply that the Saviour die again, considering that transubstantiation permits an unbloody sacrifice.

The Holy Eucharist is quite frequently kept in a monstrance outside of Mass to encourage Adoration, which is especially imperative in the current Year of the Eucharist (declared by Servant of God Pope John Paul II). Indeed, the Eucharist constitutes both the foundation and the center of all Catholic devotion. It is one of the Seven Sacraments, referred to as the Blessed Sacrament, and consequently bestows grace upon the recipient and removes venial sin. When received following Confession and preceding an act to which an indulgence is attached, It contributes to a plenary indulgence.

In Scripture, the Eucharist is prefigured by the miraculous rain of manna from Heaven (Ex. 16:13-36). Indeed, a crowd of listeners challenges Christ on this point before he delivers the Bread of Life Discourse (john 6:31), and He describes Himself as True Bread from Heaven (John 6:32). The aforementioned Bread of Life Discourse occurs in the sixth chapter of the Gospel of John, verses 30-59. Therein, Jesus promises to give His Flesh and Blood, which will give eternal life to all who receive It.

Protestantism