Venial Sin

"Venial sin, like mortal sin, goes counter to God's will, although with less serious deviation.  While it does not destroy charity, it is opposed to it and therefore diminishes its fervor and vigor, hindering its development.  This is the disastrous effect of deliberate venial sin committed with the realization that it is displeasing to God.

"Once venial sins of this kind become habitual, they decrease the soul's tendency toward God, and increase on the other hand, its leaning toward self-satisfaction and creatures.  Thus, little by little the soul loses its fervor, its sense of sin, and falls into tepidity, which is characterized by a certain indifference to venial sin.  This puts it in danger of offending God in serious matters also.  In this sense, venial sin can be compared to a disease of insidious languor, a kind of spiritual tuberculosis which undermines the organism slowly but fatally.  It is not unusual to meet souls who having at first surrendered themselves to God with sincere fervor, afterwards let themselves fall into continual carelessness, indifference, voluntary omissions and laziness, because they have given in to selfishness and sought their own comfort.  They become incapable of making the generous efforts required to advance on the way they have started.  Their spiritual life is reduced to a kind of lethargy which is not yet death, but which has none of the freshness and vigor of a strong, healthy life.  It lacks the fervor of charity, for this is continually being lessened by deliberate venial sin."  -- from Divine Intimacy, p. 297