Reception of Holy Communion
In the Catholic Church, to receive Holy Communion is to profess that the Sacred Eucharist is the Body, Blood, Soul, and Divinity of Our Lord Jesus Christ, and to profess all that the Catholic Church teaches to be true.  For this reason, only those baptized Catholics who are in a state of grace and observing the Eucharistic fast may receive Holy Communion.

A "state of grace" refers to  those who are not conscious of any grave sin which has yet to be absolved by sacramental Confession (stjoanarc.com; cf. Canon 916).

Proper observation of the Eucharistic fast is no food or drink (other than water) one hour prior to the reception of Holy Communion (Canon 919 §1), although some may prefer to observe the older discipline of three hours or from midnight (stjoanarc.com).  Those who are advanced in age, suffer from infirmity, and those who care for them, may receive Holy Communion even if they have had food or drink within the previous hour (Canon 919 §3).