14 For all these things then let us give thanks to God who loves man; and for His tender care over us, render Him a recompense, that will also be profitable to ourselves; and as regards the commandment which I so frequently discourse of to you, let us use our utmost diligence! For I will not desist from the exhortation until you are amended: seeing that what we aim at is not that we may address you seldom or frequently, but that we may continue speaking till we have persuaded you.
To the Jews when God said by the prophet, “If you fast for strife and debate, to what purpose do ye fast for me?” And by us He says to you, If you fast unto oaths and perjuries, to what purpose do ye fast? For how shall we behold the sacred Passover? How shall we receive the holy Sacrifice? How shall we be partakers of those wonderful mysteries by means of the same tongue with which we have trampled upon God's law, the same tongue with which we have contaminated the soul? For if no one would dare to receive the royal purple with filthy hands, how shall we receive the Lord's Body with a tongue that has become polluted! For the oath is of the wicked one, but the Sacrifice is of the Lord. “What communion then has light with darkness, and what concord has Christ with Belial?”
Source: Homilies on the Statues (New Advent)