"Anyone who desires to receive Christ in Eucharistic communion must be in the state of grace. Anyone aware of having sinned mortally must not receive communion without having received absolution in the sacrament of penance." Source: Catechism of the Catholic Church #1415
"When children begin to use their reason, fathers and mothers should take great pains to fill their hearts with the fear of God. This the good Queen Blanche did most earnestly by St. Louis, her son: witness her oft-repeated words, 'My son, I would sooner see you die than guilty of a mortal sin,' words [...]
"By the same charity that it enkindles in us, the Eucharist preserves us from future mortal sins. The more we share the life of Christ and progress in his friendship, the more difficult it is to break away from him by mortal sin. The Eucharist is not ordered to the forgiveness of mortal sins - [...]
"The conclusion of all this is that you be ready to die rather than commit a mortal sin, and ready to die, also, rather than commit a venial sin with deliberation. Rather endure all things, rather death itself than offend God! It would be better to let the whole world go to ruin rather than [...]