…as in the Old Testament outward acts were attended to, so in the New Testament it is not so much what is done outwardly as what is thought inwardly that is regarded with close attention, that it may be punished with searching judgment. For while the law forbids the eating of many things as being unclean, the Lord nevertheless says in the Gospel, Not that which goeth into the mouth defileth a man, but the things which come forth from the heart, these are they which defile a man (Matth. xv.11). And soon after He added in explanation, Out of the heart proceed evil thoughts (Ib. 19). Hence it is abundantly indicated that what is shewn by Almighty God to be polluted in act is that which is engendered of the root of polluted thought. Whence also Paul the Apostle says, All things are pure to the pure; but unto them that are defiled and unbelieving is nothing pure (Tit. i. 15). And immediately, to declare the cause of this defilement, he subjoins, For their mind and conscience is defiled.
St. Gregory Diologos