The doctrine of sin is central to Christianity, since its basic message is about redemption in Christ. Christian hamartiology (from Greek: ἁμαρτία, hamartia, "missing the mark, error" and -λογια, -logia, "study"), a branch of Christian theology which is the study of sin, describes sin as an act of offence against God by despising His persons and Christian biblical law, and by injuring others. In Christian views it is an evil human act, which violates the rational nature of man as well as God's nature and His eternal law. According to the classical definition of St. Augustine of Hippo sin is "a word, deed, or desire in opposition to the eternal law of God." Christian hamartiology is closely related to concepts of natural law, moral theology and Christian ethics.
Among some scholars, sin is understood mostly as legal infraction or contract violation of non-binding philosophical frameworks and perspectives of Christian ethics, and so salvation tends to be viewed in legal terms.
Other Christian scholars understand sin to be fundamentally relational—a loss of love for the Christian God and an elevation of self-love ("concupiscence", in this sense), as was later propounded by Augustine in his debate with the Pelagians. As with the legal definition of sin, this definition also affects the understanding of Christian grace and salvation, which are thus viewed in relational terms.
Early in 1997 the Vatican issued pastoral guidelines for priests to urge divorced catholics who remarry not to live in a state of sin -- meaning no sex in the new relationship. Such couples were to be invited to recongize their irregular situation, which involves a state of sin, and ask God for the grace of a true conversion.