During Mass, at the Lamb of God, the priest breaks a piece of the consecrated host and places it in the chalice. During this action, he says a prayer quietly: “May this mingling of the Body and Blood of our Lord Jesus Christ bring eternal life to us who receive it.” One meaning of this commingling of the Body and Blood of Jesus was expressed by the liturgical scholar Dom Prosper Guéranger (died 1875) when he wrote: “Its object is to show that, at the moment of Our Lord’s Resurrection, His Blood was reunited to his Body; by flowing again in his veins as before.”
Another meaning comes an early Church practice called fermentum. This practice involved bringing a particle from the consecrated host at the bishop’s Mass (called a fermentum, from the Latin word for leaven or yeast) to the parish church and mingled in the priest’s chalice as a sign of the unity of the priest’s Mass with the bishop’s. The fermentum was used brought from the bishop of one diocese to the bishop of another diocese. The receiving bishop would then consume the species at his next celebration of the Eucharist as a sign of the communion between the churches. The fermentum then is a sign of unity: between the bishop and the parish; between dioceses; and between Christ and his Church.
Now you might be asking why I am talking about this during the season of Advent and in our preparation of Christmas. St. Alphonsus Liguori gives an explanation about this commingling. He wrote: “This mingling of the holy species represents, too, the unity of divinity with humanity, which was at first effected in the womb of Mary through the incarnation of the Word, and which is renewed in the souls of the faithful when they receive him in the eucharistic Communion.”
At the preparation of the gifts (the offertory), the priest or deacon pours a little water into the chalice filled with wine and says quietly: “By the mystery of this water and wine may we come to share in the divinity of Christ who humbled himself to share in our humanity.” This prayer comes from an ancient Christmas prayer that we still use today as the Collect (opening prayer) for Christmas Mass on Christmas Day. As we prepare for Christmas this Advent, we recall the unity of the divinity and humanity of Jesus—a unity shown through the mixture of water and wine and the unity that is the Eucharist. Holy Communion is the sacrament of unity.
That Christmas Collect is a good preparation for us this Advent: “O God, who wonderfully created the dignity of human nature and still more wonderfully restored it, grant, we pray, that we may share in the divinity of Christ, who humbled himself to share in our humanity. Amen.”