Nature of original sin
… Original sin is the privation of sanctifying grace in consequence of the sin of Adam.
… original sin is distinct from the fault of Adam, it is one of its effects. But which of these effects is it? We shall examine the several effects of Adam’s fault and reject those which cannot be original sin:
(1) Death and Suffering.- These are purely physical evils and cannot be called sin. Moreover St. Paul, and after him the councils, regarded death and original sin as two distinct things transmitted by Adam.
(2) Concupiscence.- This rebellion of the lower appetite transmitted to us by Adam is an occasion of sin and in that sense comes nearer to moral evil. However, the occasion of a fault is not necessarily a fault, and whilst original sin is effaced by baptism concupiscence still remains in the person baptized; therefore original sin and concupiscence cannot be one and the same thing, as was held by the early Protestants (see Council of Trent, Sess. V, can. v).
(3) The absence of sanctifying grace in the new-born child is also an effect of the first sin, for Adam, having received holiness and justice from God, lost it not only for himself but also for us (loc. cit., can. ii). If he has lost it for us we were to have received it from him at our birth with the other prerogatives of our race. Therefore the absence of sanctifying grace in a child is a real privation, it is the want of something that should have been in him according to the Divine plan. If this favour is not merely something physical but is something in the moral order, if it is holiness, its privation may be called a sin. But sanctifying grace is holiness and is so called by the Council of Trent, because holiness consists in union with God, and grace unites us intimately with God. Moral goodness consists in this, that our action is according to the moral law, but grace is a deification, as the Fathers say, a perfect conformity with God who is the first rule of all morality. (See GRACE.) Sanctifying grace therefore enters into the moral order, not as an act that passes but as a permanent tendency which exists even when the subject who possesses it does not act; it is a turning towards God, conversio ad Deum. Consequently the privation of this grace, even without any other act, would be a stain, a moral deformity, a turning away from God, aversio a Deo, and this character is not found in any other effect of the fault of Adam. This privation, therefore, is the hereditary stain.