An extract from 'Theory and Practice of the Confessional by Prof. Caspar E. Schieler, D.D.'
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But there is an obligation to avoid putting off for a long time one's conversion, and hence an act of perfect contrition after mortal sin, because a man in the state of mortal sin is in the greatest danger of falling into other mortal sins, since he has not strength enough to vanquish severe temptations and to withstand the violence of his passions, and since, as St. Gregory the Great says, the unrepented mortal sins which burden his soul draw him by their weight into other worse sins.

"Without sanctifying grace it is not possible to refrain long from mortal sin," says St. Thomas; the sinner might, if he wished, have the necessary moral strength to overcome temptation and to resist his passions; he might curb them by the divine power of grace; but there is the law of the distribution of God's graces, that God gives only to those who love Him efficacious grace, and while a man persists of his own free will in the state of sin and enmity with God, he equivalently expresses his contempt of grace and so makes himself unworthy of it. As God is ever pouring richer and richer graces on those who make good use of them and cooperate with them, so He withdraws them from those who neglect and resist them.