Roman Catholic theology generally teaches that at the Words of Institution the bread is changed into the Body of Christ (see transubstantiation), whereas Eastern Christian theology generally views the epiclesis as the point at which the change occurs. Some Protestants believe transignification occurs at the Words of Institution.
The word "host" is derived from the Latin hostia, which means "sacrificial victim". The term can be used to describe the bread both before and after consecration, though it is more correct to use it after consecration. Prior to consecration, the term "altar bread" is preferred. The sacrifice of Jesus according to Christian theology puts an end to the need for animal sacrifice as had been practiced in the Jerusalem Temple and all blood sacrifice once-for-all. However the word was retained to describe the bread of Eucharist as a liturgical representation of the Christ's sacrifice.
With the exception of Churches of the Armenian Rite, Orthodox Churches and Eastern Catholic Churches use leavened bread for the Eucharist. Thus, the sacramental bread symbolizes the Resurrected Christ. The hostia or sacramental bread, known as prosphorá or a πρόσφορον (prósphoron, "offering") may be made out of only four ingredients: fine (white) wheat flour, pure water, yeast, and salt. Sometimes holy water will be either sprinkled into the dough or on the kneading trough at the beginning of the process.
Because leaven is symbolic of sin, the Armenian Catholic Church and the Armenian Apostolic Orthodox Church traditionally offer unleavened bread (although it is distinctively different from the kind used by the Roman Catholic Church) to symbolize the sinlessness of Christ.
The baking may only be performed by a believing Orthodox Christian in good standing—having preferably been recently to Confession, and is accompanied by prayer and fasting. Before baking, each loaf is formed by placing two disks of dough, one on top of the other, and stamping it with a special liturgical seal. The prosphora should be fresh and not stale or moldy when presented at the altar for use in the Divine Liturgy. Often several prosphora will be baked and offered by the faithful, and the priest chooses the best one for the Lamb (Host) that will be consecrated. The remaining loaves are blessed and offered back to the congregation after the end of the Divine Liturgy (Eucharist); this bread is called the antidoron (Greek: αντίδωρον, antídōron), i.e. a "gift returned", or "in place of the gifts".