“The devil tries to soil and defile every good thing a man would do by intermingling with it his own seeds in the form of self-esteem, presumption, complaint, and other things of this kind, so that what we do is not done for God alone, or with a glad heart. Abel offered a sacrifice to God of the fat and firstlings of his flock, while Cain offered gifts of the fruits of the earth, but not of the firstfruits; and that is why God looked with favor on Abel’s sacrifices, but paid no attention to Cain’s gifts (cf. Gen. 4:3-5). This shows us that it is possible to do something good in the wrong way…”
(St Makarios of Egypt, homily in the Philokalia)