Having considered both sin in general and original sin, we now begin a closer examination of actual sin, the sin one personally commits. This chapter considers the distinction among sins based on their seriousness, especially the distinction between mortal and venial sins. It also deals with sins of thought. Chapters sixteen and seventeen take up in detail problems about the conditions for mortal sin. Chapter eighteen describes the dynamics by which a life of sin can decline from imperfection to everlasting death.
Not all sins are equally serious. On a scriptural basis, the Church distinguishes venial sins from mortal sins. Because Christian morality primarily concerns the existential self—rightness of the heart—sins of thought are important. Like other sins, sins of thought can be mortal only if one makes a definite choice.