Due to the intimate connection of the Sacrament of Holy Communion to our life in Christ, Catholics who wish to receive Communion must be in a state of grace that is, free of any grave or mortal sin before receiving it, as St. Paul explained in 1 Corinthians 11:27-29. Otherwise, as he warns, we receive the sacrament unworthily, and we “eateth and drinketh damnation” to ourselves.
When we are aware of having committed a mortal sin, we must participate in the Sacrament of Confession first. The Church sees the two sacraments as connected, and urges us, when we can, to join frequent Confession with frequent Communion. In order to receive Communion, we must also abstain from food or drink (except for water and medicine) for one hour beforehand.