According to Aquinas, the difference is not as great as one might think. Let us consider that our access to Christ’s real and substantial presence in the Holy Eucharist is by our faith and charity directed toward this mystery. Our eating of Christ’s Body and Blood is an eating by faith and love, since the appearances of bread and wine are consumed. These appearances are the sign of the presence of his Body and Blood, but we obviously eat and drink his Body and Blood by our faith in his presence and our desire to be united to him in charity, not by our physical chewing and swallowing. These natural functions, like the natural outward forms of bread and wine, are the signs of our deeper, inward spiritual eating of him.
Our Lord himself, in the sixth chapter of John’s Gospel, tells us that his presence can be understood only in a spiritual and not in a fleshly way. This does not mean that his Body and Blood are not as really present as any other thing offered to our senses; it means that that we lay hold of this mystery by faith and love, not by bodily functions. So, in both a visible, sacramental Communion and in a spiritual one, we feed on the Body and Blood of Christ by faith and love, and we receive the same effects in either case.
It remains true, however, that with the sacraments the grace is greater and more precise if we participate in the sacrament in fact and not only in desire. So our spiritual Communion is always a desire to receive the Lord sacramentally, but it is still in virtue of the sacrament that we receive the grace of a spiritual Communion. A spiritual Communion is a genuine, though less sacramentally perfect, sharing in the Body and Blood of the Lord.
Holy Church grants a partial indulgence for each spiritual Communion, and we can apply this indulgence to the departed, who cannot receive Communion any more. What a wonderful way to assist those who long to see God in the banquet of heaven!