Taken from I’m Glad You Asked!; St. Charles Borromeo Catholic Church, Picayune, MS. Access the entire document at http://www.scborromeo.org/glad.htm
Q. Is it a mortal sin to receive Communion without believing that it is truly the body, blood, soul and divinity of Christ?
As adopted by the National Conference of Catholic Bishops and printed in every missalette:
"Catholics fully participate in the celebration of the Eucharist when they receive Holy Communion in fulfillment of Christ's command to eat His Body and drink His Blood. In order to be properly disposed to receive Communion, communicants should not be conscious of grave sin, have fasted for one hour, and seek to live in charity and love with their neighbors. Persons conscious of grave sin must first be reconciled with God and the Church through the sacrament of Penance. A frequent reception of the sacrament of Penance is encouraged for all."
The Catholic belief is that Holy Communion is in fact the body, blood, soul and divinity of our Lord Jesus Christ. This belief will be more thoroughly discussed in the chapter on the Sacrament of the Eucharist. As to whether receiving Holy Communion without believing this, we look to what Saint Paul says as he writes to us in 1 Corinthians 11:23-29:
"For I received from the Lord what I also passed on to you: The Lord Jesus, on the night he was betrayed, took bread, and when he had given thanks, he broke it and said, 'This is my body, which is for you; do this in remembrance of me.' In the same way, after supper he took the cup, saying, 'This cup is the new covenant in my blood; do this, whenever you drink it, in remembrance of me.' For whenever you eat this bread and drink this cup, you proclaim the Lord's death until he comes. Therefore, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of sinning against the body and blood of the Lord. A man ought to examine himself before he eats of the bread and drinks of the cup. For anyone who eats and drinks without recognizing the body of the Lord eats and drinks judgement on himself." (NIV) [emphasis added]
As you can see in the last sentence, it shows the harshest of penalties for receiving the Eucharist without recognizing (discerning) the body of the Lord. The Eucharist is not ordinary food and drink to nourish the body; it is supernatural food and drink to nourish the soul. Receiving it as other than the body, blood, soul and divinity of our Lord Jesus desecrates the sacrament. It is saying "I believe Jesus lied when He said 'This is My Body . . . This is My Blood'." Jesus died on the cross so we could all have this universal meal to share as members of Gods' one family. By denying it, one makes Jesus' sacrifice on the cross meaningless for them.