32 Manifestly also in the Gospel we find the mouth of the heart: so that in one place the Lord is found to have mentioned the mouth both of the body and of the heart, where he says, “Are ye also yet without understanding? Do ye not yet understand, that whatsoever enters in at the mouth, goes into the belly, and is cast out into the draught? But those things which proceed out of the mouth come forth from the heart, and they defile the man. For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies: these are the things which defile a man.” Here if you understand but one mouth, that of the body, how will you understand, “Those things which proceed out of the mouth, come forth from the heart;” since spitting also and vomiting proceed out of the mouth?
Unless perhaps a man is but then defiled when he eats anything unclean, but is defiled when he vomits it up. But if this be most absurd, it remains that we understand the mouth of the heart to have been expounded by the Lord, when He says, “The things which proceed out of the mouth, come forth from the heart.” For being that theft also can be, and often is, perpetrated with silence of the bodily voice and mouth; one must be out of his mind so to understand it as then to account a person to be contaminated by the sin of theft, when he confesses or makes it known, but when he commits it and holds his peace, then to think him undefiled.
But, in truth, if we refer what is said to the mouth of the heart, no sin whatever can be committed tacitly: for it is not committed unless it proceed from that mouth which is within.
Source: On Lying (New Advent)