The earliest Patristic literature witnesses to a keen sensitivity to the obligation to avoid all defiling thoughts. Adultery was the chief of such unclean and deadly thoughts, but any thoughts against chastity, as well as anger, greed, or jealousy were also defiling, as Jesus had stated already. However, other fathers understood that these were not the only obstacles to holiness of life. Purity of heart was not only a matter of eliminating desires and thoughts of disordered passion, but also demanded right thoughts about God, Jesus and creation. More explicitly, St. Irenaeus understands the need for true faith to attain to a pure heart.
Action then is perfected by faith.
Faith must be put into practice by study and meditation as the same Father points out in another text in his work against heretics who threaten the true faith. Irenaeus comments further that it is not only the vice of impurity but also the lack of faith that renders certain persons carnal and animal-like, and constitutes an obstacle to their receiving the Holy Spirit. Later on St. Gregory of Nyssa was to make the same point more strikingly. Cassian also was to take up this same theme, considering that for the monk any thought that was idle or concerned with matters not pertaining to the search for God was a form of spiritual adultery. Pertinent to our inquiry, though, is the fact that St. Gregory here considers these various passions not only as moral defects but also precisely as opposed to sound doctrine.