The path of holiness is a source of peace and joy, given to us by the Spirit. At the same time, it demands that we keep “our lamps lit” (Lk 12:35) and be attentive. “Abstain from every form of evil” (1 Thess 5:22). “Keep awake” (Mt 24:42; Mk 13:35). “Let us not fall asleep” (1 Thess 5:6). Those who think they commit no grievous sins against God’s law can fall into a state of dull lethargy. Since they see nothing serious to reproach themselves with, they fail to realize that their spiritual life has gradually turned lukewarm. They end up weakened and corrupted.
Spiritual corruption is worse than the fall of a sinner, for it is a comfortable and self-satisfied form of blindness. Everything then appears acceptable: deception, slander, egotism and other subtle forms of self-centredness, for “even Satan disguises himself as an angel of light” (2 Cor 11:14). So Solomon ended his days, whereas David, who sinned greatly, was able to make up for disgrace. Jesus warned us against this self-deception that easily leads to corruption. He spoke of a person freed from the devil who, convinced that his life was now in order, ended up being possessed by seven other evil spirits (cf. Lk 11:24-26). Another biblical text puts it bluntly: “The dog turns back to his own vomit” (2 Pet 2:22; cf. Pr 26:11).
Gaudete et Exsultate, 164-165