When the Spirit descended in tongues of fire, it was to make the waiting group into the mystical Body of Christ in a way analogous to that in which the descent of the Spirit upon Mary at her Annunciation had formed the natural body of Christ in her womb. Nevertheless, although the Mystical Body came into being by this new descent of the Spirit, there was not a new incarnation, Christ was not becoming man a second time, he was not assuming a new nature; the human nature which he had taken from his mother, in which he had died for our sins and risen again for our justification, was being made present under a new mode. There are not, strictly speaking, two bodies of Christ, a natural and a mystical, but one body of Christ which is manifested in two forms.
Nor does the story end here, for that part of the Mystical Body which is on earth needs to be continually nourished and sustained, as Christ’s natural body did before its glorification. It is through the Eucharistic Body of the Blessed Sacrament that this takes place. Here again, there is not a new incarnation, but in the Eucharist the human nature which Christ took from his mother is made present in yet another form, a form through which that part of the Mystical Body which is still in via on earth is repeatedly sustained and renewed.