a) Only baptized Catholics
“The sacraments of the Church are open only to the faithful who have received baptism.” (Code of Canon Law CIC can. 912)
b) The faithful must be in a state of grace
This means: No mortal sin.
“Anyone who is aware of having committed a grave sin must not receive Holy Communion without prior sacramental confession.” (CIC can. 916; cf. Catechism of the Catholic Church CCC 1385)
This is an absolute norm.
Deliberately receiving Communion in a state of mortal sin would itself be a sacrilege (cf. 1 Cor 11:27–29).
c) Faith in the real presence of Christ
Receiving requires faith in the real presence of the Lord in the Eucharistic species (cf. CCC 1374–1381).