13 Or haply is it so, that he who plots in this way to find out Priscillianists, denies not Christ, forasmuch as with his mouth he utters what with his heart he believes not? As if truly (which I also said a little above) when it was said, “With the heart man believes unto righteousness,” it was added to no purpose, “with the mouth confession is made unto salvation?” Is it not so that almost all who have denied Christ before the persecutors, held in their heart what they believed of Him?
And yet, by not confessing with the mouth unto salvation, they perished, save they which through penitence have lived again? Who can be so vain, as to think that the Apostle Peter had that in his heart which he had on his lips when he denied Christ? Surely in that denial he held the truth within and uttered the lie without. Why then did he wash away with tears the denial which he uttered with his mouth, if that sufficed for salvation that with the heart he believed? Why, speaking the truth in his heart, did he punish with so bitter weeping the lie which he brought forth with his mouth, unless because he saw it to be a great and deadly evil, that while with his heart he believed unto righteousness, with his mouth he made not confession unto salvation?
Source: To Consentius: Against Lying (New Advent)