We receive Holy Communion to follow the command of the Lord at the Last Supper when he instituted the Holy Eucharist.
Catholics believe that the Holy Eucharist is the actual Flesh and Blood of Jesus Christ because that is what Christ said It is, “This is my body……This is my blood” (Matt. 26:26-28; Luke 22:19-20; Mark 14:22-24). Also, because Christ said, “Unless you eat the flesh of the Son of man, and drink his blood, you have no life in you….” (John 6:48-52; 54-56), and because this is what the Apostle believed, “The cup of blessing which we bless, is it not a participation in the blood of Christ? The bread which we break, is it not a participation in the body of Christ?” (1 Cor. 10:16)