Pope John Paul has a beautiful paragraph in which he relates Mary’s fiat to the Amen every believer says when receiving Communion. Mary was asked to believe that the one whom she conceived "through the Holy Spirit" was "the Son of God." In the Eucharistic mystery we are asked to believe that the one whom she conceived is Son of God and son of Mary. We are asked to believe that Jesus is present in his full humanity and divinity under the signs of bread and wine.
John Paul says that Mary anticipated in the mystery of the Incarnation the Church’s Eucharistic faith. He writes, "When, at the Visitation, she bore in her womb the Word made flesh, she became in some way a tabernacle in which the Son of God, still invisible to our human gaze, allowed himself to be adored by Elizabeth, radiating his light as it were through the eyes and voice of Mary" (55.3).
The pope gives us a "re-reading" of the Magnificat in a Eucharistic key. The Eucharist like the Canticle of Mary is first and foremost praise and thanksgiving. When Mary exclaims: "My soul magnifies the Lord and my spirit rejoices in God my Savior," she already bears Jesus in her womb (like a living tabernacle). She praises God through Jesus, but she also praises him in Jesus and with Jesus. This is itself the true Eucharistic attitude. At the same time, Mary recalls the wonders worked by God in salvation history in fulfillment of the promise once made to Abraham and our spiritual ancestors.She proclaims the wonder that surpasses them all, the redemptive Incarnation. Lastly, the Magnificat reflects the eschatological tension of the Eucharist. Every time the Son of God comes to us in the “poverty” of the sacramental signs of bread and wine, the seeds of that new history wherein the "mighty are put down from their thrones and those of low degree are exalted," take root in the world. The Magnificat expresses Mary's spirituality, and there is nothing greater than this spirituality for helping us to experience the mystery of the Eucharist. -- Archbishop Sean O'Malley