4 Therefore, although wicked men press forward to keep the feast, and as at a feast praise God, and intrude into the Church of the saints, yet God expostulates, saying to the sinner, 'Why do you talk of My ordinances?' And the gentle Spirit rebukes them, saying, 'Praise is not comely in the mouth of a sinner. ' Neither has sin any place in common with the praise of God; for the sinner has a mouth speaking perverse things, as the Proverb says, 'The mouth of the wicked answers evil things.' For how is it possible for us to praise God with an impure mouth? Since things which are contrary to each other cannot coexist. For what communion has righteousness with iniquity? Or, what fellowship is there between light and darkness? So exclaims Paul, a minister of the Gospel.
Thus it is that sinners, and all those who are aliens from the Catholic Church, heretics, and schismatics, since they are excluded from glorifying (God) with the saints, cannot properly even continue observers of the feast. But the righteous man, although he appears dying to the world, uses boldness of speech, saying, 'I shall not die, but live, and narrate all Your marvelous deeds. ' For even God is not ashamed to be called the God of those who truly mortify their members which are upon the earth, but live in Christ; for He is the God of the living, not of the dead. And He by His living Word quickens all men, and gives Him to be food and life to the saints; as the Lord declares, 'I am the bread of life.' The Jews, because they were weak in perception, and had not exercised the senses of the soul in virtue, and did not comprehend this discourse about bread, murmured against Him, because He said, 'I am the bread which came down from heaven, and gives life unto men. '
Source: Letters (New Advent)