Sacrament of the Holy Eucharist
"I am the living bread that came down from heaven; if any one eats of this bread, he will live for ever; ... he who eats my flesh and drinks my blood has eternal life and ... abides in me, and I in him" (Jn. 6:51, 54, 56.)
Catholics who are in a state of grace, that is without mortal sin, are encourage to take communion as often as possible or at least on Sundays and on Holy days of obligation.
Those who are conscious of having committed a mortal sin are asked to reconcile their relationship with God through the Sacrament of Reconciliation (Confession) before taking the Eucharist.