3 For if any of those who are present should think to tempt God's grace, he deceives himself, and knows not its power. Keep your soul free from hypocrisy, O man, because of Him who searches hearts and reins. For as those who are going to make a levy for war examine the ages and the bodies of those who are taking service, so also the Lord in enlisting souls examines their purpose: and if any has a secret hypocrisy, He rejects the man as unfit for His true service; but if He finds one worthy, to him He readily gives His grace. He gives not holy things to the dogs; but where He discerns the good conscience, there He gives the Seal of salvation, that wondrous Seal, which devils tremble at, and Angels recognise; that the one may be driven to flight, and the others may watch around it as kindred to themselves. Those therefore who receive this spiritual and saving Seal, have need also of the disposition akin to it. For as a writing-reed or a dart has need of one to use it, so grace also has need of believing minds.
Source: Catechetical Lectures (New Advent)