It's a matter of honesty

Archbishop Chaput addresses what is means to be "in communion", what it means to make the claim that one is Catholic.

It's a matter of honesty: to receive Communion, we need to be in communion

When we sin by theft, lying, adultery, pride, gossip, anger, envy, callousness to the poor, pornography or indifference, we do not live "in keeping with what Christ taught." We remove ourselves, by our actions, from friendship with God. That means we need to turn back to the sacrament of penance before we receive Communion. In fact, many of us today need a deeper devotion to confession simply to regain a basic understanding of grace and sin.

Likewise, if we ignore or deny what the Church teaches, or refuse to follow what she teaches, we are not "in communion" with the Catholic faith. We separate ourselves from the community of believers. If we receive Communion anyway, we engage in a lie.

Claiming to be Catholic and then rejecting Catholic teaching is an act of dishonesty and a lack of personal integrity. Worse, if we then receive Communion, we violate every Catholic who does believe and does strive to live the faith fully and unselfishly. And that compounds a sin against honesty with a sin against justice and charity. Again, as Justin Martyr said: "No one may take part (in the Eucharist) unless he believes what we teach is true."

Archbishop's Column here.