The reason has to do with the definition of bread. Communion is a rite that traces back to the early Catholic Church. In St. Paul’s First Epistle to the Corinthians (in the Bible’s New Testament), which was written around A.D. 53 or 54, Paul lays out the ritual and its roots in the Last Supper, Jesus Christ’s final meal before the Crucifixion:
“For I have received from the Lord that which also I delivered unto you: that the Lord Jesus, the same night in which He was betrayed, took break,” reads the King James translation of the passage, “and when He had given thanks, He broke it and said, ‘Take, eat; this is My body which is broken for you: this do in remembrance of Me.”
Under Catholic doctrine, consecrated bread and wine transubstantiate, or become the literal body and blood of Christ. Thus, the purity of the bread and wine are important, said Rev. Andrew Menke, the executive director of the Secretariat of Divine Worship for the U.S. Conference of Catholic Bishops.