Unleavened Bread | Christ The Leaven

Catholics, specifically we of the Latin (Roman) Rite, use unleavened bread in the Eucharist (Holy Communion) primarily because it is acknowledged to be the type of bread that Jesus used at the Last Supper, which coincided with the Jewish Passover Seder, where unleavened bread is traditionally used. The practice acknowledges the connection of the Mass to the Last Supper and Jesus's sacrifice.

Historical Context

During the time of the Last Supper, the Jewish Passover meal (Seder) was being celebrated, and Jewish law mandated the use of unleavened bread (matzah). 

Symbolism

Unleavened bread, lacking leaven (yeast), symbolizes purity and the absence of sin. 

Tradition and Continuity

By using unleavened bread, the Catholic Church maintains a link to the historical Last Supper and emphasizes the continuity of the Eucharistic sacrifice from that event.

Eastern Catholic Churches

While the Eastern Catholic Churches also celebrate the Eucharist, some of them use leavened bread, reflecting different liturgical traditions and interpretations of the Last Supper.

The Offertory | Form One | Divine Worship: the Missal

Receive, O holy Father, almighty and everlasting God, this spotless host,... .

The Roman Canon | Divine Worship: the Missal

THEREFORE, most merciful Father, we humbly pray thee, through Jesus Christ thy Son our Lord, and we ask, that thou accept and bless these  gifts, these offerings, these holy and unblemished sacrifices.

Transubstantiation

The Catholic Church believes that during the consecration in the Eucharist, the bread and wine are transformed into the Body and Blood of Christ (transubstantiation). The type of bread (leavened or unleavened) does not change the belief in the real presence of Christ.

Christ is the leaven. Christ is the yeast. He was made sin for us.

2 Corinthians 5:21 | For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.

Jesus is the living bread from heaven (St John 6:35). Jesus, Himself, supplies the leaven for the bread that becomes His very Body.

1 Corinthians 5:7–8 Biblia Sacra Vulgata

Expurgate vetus fermentum, ut sitis nova conspersio, sicut estis azymi. Etenim Pascha nostrum immolatus est Christus. Itaque epulemur: non in fermento veteri, neque in fermento malitiae et nequitiae: sed in azymis sinceritatis et veritatis.

1 Corinthians 5:7–8

Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us: Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.

1 Corinthians 5:7-8 RSVCE

Cleanse out the old leaven that you may be a new lump, as you really are unleavened. For Christ, our paschal lamb, has been sacrificed. Let us, therefore, celebrate the festival, not with the old leaven, the leaven of malice and evil, but with the unleavened bread of sincerity and truth.

The Pascha Nostrum | Divine Worship: the Missal

The People kneel. As the Priest takes the Host and breaks it, he sings or says: (Alleluia.) Christ our Passover is sacrificed for us;

People: Therefore let us keep the feast. (Alleluia.)

The use of unleavened bread for Mass has been the continuous practice. The language of the Mass, the Roman Canon, which is among the most ancient of canons of the undivided Church, describes the host (bread for Mass) in a way that affirms that it is unleavened. The rubrics that guide the celebration of Mass also reinforce the acknowledgement of the kind of bread being used.

Rome had a sizable Jewish population, estimated to be several thousand people, and it would have been entirely logical for the community to understand the significance of the use of unleavened bread and, as a result, the connection between the Lord's Supper, i.e., the Mass, and the Last Supper and Jesus' sacrifice.