Jesus multiplies the loaves to give a tangible example of miraculous bread before then teaching the dogma of the Eucharist in the Bread of Life Discourse. He prepares us for his teaching by showing us a miracle.
Similarly, the Church prepares us for belief in the Eucharist by the rituals she uses during the Mass and in adoration of the Eucharist outside of Mass.
Consider: In the Mass, we know by example that the bread is changed into the Eucharist at the moment of the words of consecration, because it is after this moment that the priest holds up the Host for us to adore the Lord. Likewise, we know that Jesus is living and active in the Eucharist, because we ask him to forgive us our sins as we look to the Host and say, "Lord, I am not worthy."
Likewise, outside of Mass, we consider how the rite of Benediction and the Eucharistic Procession teach us that Jesus is a priest in the Eucharist. From the Sacred Host, he blesses his people and intercedes for us!
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