St Cyril of Alexandria wrote in his commentary on St John’s Gospel:
Our Lord Jesus Christ did not pray only for the twelve disciples. He prayed for all in every age whom their exhortation would persuade to become holy by believing and to be purified by sharing in the Holy Spirit. “May they all be one, he prayed. As you Father are in me and I am in you, may they also be one in us.”
...By his own wisdom and the Father's counsel he devised a way of bringing us all together and blending us into a unity with God and one another, even though the differences between us give us each in both body and soul a separate identity. For in holy communion he blesses with one body, which is his own, those who believe in him, and makes them one body with himself and one another. Who could separate those who are united to Christ through that one sacred body, or destroy their true union with one another? If we all share one loaf we all become one body, for Christ cannot be divided.
So it is that the Church is the body of Christ and we are its members. For since we are all united to Christ through his sacred body, having received that one indivisible body into our own, our members are not our own but his.