Our relationship to God needs not only the inward aspect; it also needs to be expressed. And as well as speech, singing and silence, standing, sitting and kneeling, expression also calls for this celebratory walking along together in the community of the faithful, together with the God in whom we believe.
In the Christian liturgy itself we can identify two elements which gave rise to the Corpus Christi procession. The liturgy of Holy Week, in which the Church reenacts the drama of the last week of Jesus’ life, presents two “processional” paths found in the sacred events themselves, namely, the procession of palms and Jesus’ ascent to the Mount of Olives after the institution of the Eucharist.
In the one he enters the Holy City in triumph; in the other he goes from it in prayer, into the darkness of night, into betrayal and death. There is a close relationship between these two processions: Jesus enters the city to cleanse the Temple, symbolically destroying it and thus incurring his death. This in turn is the inner precondition for his giving of himself in instituting the Eucharist and this opening the new Temple of his love. Again, in sharing himself in the Eucharist, he is anticipating his death and looking forward to the Resurrection. But his departure from the city into the Passover night is a departure from the peaceful and protected sphere of salvation into the realm of death. Very early on, the liturgy began to enact these processions in a solemn manner.
In certain parts of France in the eleventh century the Blessed Sacrament was carried along in the procession of palms: it was a case of going beyond mere historical remembrance and of accompanying Christus Victor on his triumphal entry into his house to take possession of it once again. Essentially, the Holy Thursday procession is an accompanying of the Host, a walking with the Lord as he goes to deliver himself up for us. All this must be peripheral in Holy Week, but Corpus Christi brings these partial elements of the Easter mystery into the center and makes them into a special great feast. What was ambivalent on Palm Sunday, overshadowed by the darkness of the Cross, takes place publicly and on a grand scale at Corpus Christi in the joy of the Resurrection.
From: The Feast of Faith, pp.132-34