4 Now what the difference is between being deceived, and lying, I will briefly state. He is deceived who thinks what he says to be true, and therefore says it, because he thinks it true. Now if this which he that is deceived says, were true, he would not be deceived; if it were not only true, but he also knew it to be true, he would not lie. He is deceived then, in that it is false, and he thinks it true; but he only says it because he thinks it true. The error lies in human infirmity, not in the soundness of the conscience.
But whosoever thinks it to be false, and asserts it as true, he lies. See, my Brethren, draw the distinction, you who have been brought up in the Church, instructed in the Lord's Scriptures, not uninformed, nor simple, nor ignorant men. For there are among you men learned and erudite, and not indifferently instructed in all kinds of literature; and with those of you who have not learned that literature which is called liberal, it is more that you have been nourished up in the word of God.
If I labour in explaining what I mean, do ye aid me both by the attention of your hearing, and the thoughtfulness of your meditations. Nor will you aid, unless you are aided. Wherefore pray we mutually for one another, and look equally for our common Succour. He is deceived, who whereas what he says is false, thinks it to be true; but he lies, who thinks a thing to be false, and gives it out as true, whether it be true or false. Observe what I have added, “whether it be true or false;” yet he who thinks it to be false, and asserts it as true, lies; he aims to deceive.
For what good is it to him, that it is true? He all the while thinks it false, and says it as if it were true. What he says is true in itself, it is in itself true; with regard to him it is false, his conscience does not hold that which he is saying; he thinks in himself one thing to be true, he gives out another for truth. His is a double heart, not single; he does not bring out that which he has in it. The double heart has long since been condemned. “With deceitful lips in a heart and a heart have they spoken evil things.” Had it been enough to say, “in the heart have they spoken evil things,” where is the “deceitful lips”? What is deceit?
When one thing is done, another pretended. Deceitful lips are not a single heart; and because not a single heart, therefore “in a heart and a heart;” therefore “in a heart” twice, because the heart is double.
Source: Sermons on Selected Lessons of the New Testament (New Advent)