St. Thomas Aquinas drew an intriguing comparison between the life of the body and the life of the soul, suggesting that each of the sacraments corresponds to an event or element of our natural existence. Just as baptism corresponds to birth, and confirmation to the passage to adulthood, he called the Eucharist
spiritual food and spiritual medicine,
corresponding spiritually to the nourishment and healing brought about by food in our bodily lives.
Catholics believe that not all sins carry the same weight or have the same effect on our souls, and traditionally make a clear distinction between “mortal sins,” which separate a Christian from the life of grace, and “venial sins,” which do not. In the text of his First Letter, St. John already speaks of a sin that leads to death (pros thanaton), as opposed to a sin that does not lead to death (me pros thanaton) - referring here to a spiritual death.
Moreover, in speaking of the Eucharist, St. Paul writes that whoever “eats the bread or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord” and then adds that a person “should examine himself, and so eat the bread and drink the cup.” (1 Cor 11:27-28)
Clearly, then, for St. Paul, though no one is truly worthy to receive the Eucharist, there is a sort of unworthiness that needs to be addressed before approaching the table of the Lord. The Catholic Church has traditionally understood this sort of unworthiness to be a state of having committed a mortal sin.
The way that this is expressed in the Catholic Catechism is that anyone “who is aware of having committed a mortal sin must not receive Holy Communion … without having first received sacramental absolution.” (No. 1457)
Aquinas, carrying on his analogy between the spiritual life of the soul and the natural life of the body, explains this by saying that the Eucharist is food, but food only helps the living. The one who commits a mortal sin
is not alive spiritually, and so he ought not to eat the spiritual
nourishment, since nourishment is confined to the living.
While Holy Communion, like bodily food, strengthens the weak and helps heal the infirm (which Aquinas equates with venial sin), it does not raise the dead. This, he said, is what the sacrament of penance or reconciliation is for.
The Church sees the state of grace - the life of the soul - to be a minimum condition or a low bar, so to speak, to be able to fruitfully receive the Eucharist. But that should be the only reason. As Aquinas says further:
Mortal sin alone necessarily prevents anyone from partaking of this