The Church believes it has a responsibility to defend the indissolubility of marriage.
In cases where a marriage does break down, and there is reason for doubt about the quality of the consent that established the marriage, there is the process of annulment.
When people do not go through the process of annulment, and remarry after a divorce, defending the indissolubility of marriage means presuming the validity of the first marriage. This is why such people are not permitted to receive Communion.
So the Catholic Church decides who is and who is not worthy of communion between the individual and God. ok then.