Objection 1. It would seem that the first movements of the sensuality in unbelievers are mortal sins. For the Apostle says (Romans 8:1) that "there is... no condemnation to them that are in Christ Jesus, who walk not according to the flesh": and he is speaking there of the concupiscence of the sensuality, as appears from the context (Romans 7). Therefore the reason why concupiscence is not a matter of condemnation to those who walk not according to the flesh, i.e. by consenting to concupiscence, is because they are in Christ Jesus. But unbelievers are not in Christ Jesus. Therefore in unbelievers this is a matter of condemnation. Therefore the first movements of unbelievers are mortal sins.
Objection 2. Further Anselm says (De Gratia et Lib. Arb. vii): "Those who are not in Christ, when they feel the sting of the flesh, follow the road of damnation, even if they walk not according to the flesh." But damnation is not due save to mortal sin. Therefore, since man feels the sting of the flesh in the first movements of the concupiscence, it seems that the first movements of concupiscence in unbelievers are mortal sins.
Objection 3. Further, Anselm says (De Gratia et Lib. Arb. vii): "Man was so made that he was not liable to feel concupiscence." Now this liability seems to be remitted to man by the grace of Baptism, which the unbeliever has not. Therefore every act of concupiscence in an unbeliever, even without his consent, is a mortal sin, because he acts against his duty.
On the contrary, It is stated in Acts 10:34 that "God is not a respecter of persons." Therefore he does not impute to one unto condemnation, what He does not impute to another. But he does not impute first movements to believers, unto condemnation. Neither therefore does He impute them to unbelievers.
I answer that, It is unreasonable to say that the first movements of unbelievers are mortal sins, when they do not consent to them. This is evident for two reasons. First, because the sensuality itself could not be the subject of mortal sin, as stated above (Question 79, Article 4). Now the sensuality has the same nature in unbelievers as in believers. Therefore it is not possible for the mere movements of the sensuality in unbelievers, to be mortal sins. Secondly, from the state of the sinner. Because excellence of the person of the person never diminishes sin, but, on the contrary, increases it, as stated above (Question 73, Article 10). Therefore a sin is not less grievous in a believer than in an unbeliever, but much more so. For the sins of an unbeliever are more deserving of forgiveness, on account of their ignorance, according to 1 Timothy 1:13: "I obtained the mercy of God, because I did it ignorantly in my unbelief": whereas the sins of believers are more grievous on account of the sacraments of grace, according to Hebrews 10:29: "How much more, do you think, he deserveth worse punishments... who hath esteemed the blood of the testament unclean, by which he was sanctified?"
Reply to Objection 1. The Apostle is speaking of the condemnation due to original sin, which condemnation is remitted by the grace of Jesus Christ, although the "fomes" of concupiscence remain. Wherefore the fact that believers are subject to concupiscence is not in them a sign of the condemnation due to original sin, as it is in unbelievers.
In this way also is to be understood the saying of Anselm, wherefore the Reply to the Second Objection is evident.
Reply to Objection 3. This freedom from liability to concupiscence was a result of original justice. Wherefore that which is opposed to such liability pertains, not to actual but to original sin.
Source: Summa Theologica (New Advent)