Objection 1. It would seem that the unbelief of heathens or pagans is graver than other kinds. For just as bodily disease is graver according as it endangers the health of a more important member of the body, so does sin appear to be graver, according as it is opposed to that which holds a more important place in virtue. Now that which is most important in faith, is belief in the unity of God, from which the heathens deviate by believing in many gods. Therefore their unbelief is the gravest of all.
Objection 2. Further, among heresies, the more detestable are those which contradict the truth of faith in more numerous and more important points: thus, the heresy of Arius, who severed the Godhead, was more detestable than that of Nestorius who severed the humanity of Christ from the Person of God the Son. Now the heathens deny the faith in more numerous and more important points than Jews and heretics; since they do not accept the faith at all. Therefore their unbelief is the gravest.
Objection 3. Further, every good diminishes evil. Now there is some good in the Jews, since they believe in the Old Testament as being from God, and there is some good in heretics, since they venerate the New Testament. Therefore they sin less grievously than heathens, who receive neither Testament.
On the contrary, It is written (2 Peter 2:21): "It had been better for them not to have known the way of justice, than after they have known it, to turn back." Now the heathens have not known the way of justice, whereas heretics and Jews have abandoned it after knowing it in some way. Therefore theirs is the graver sin.
I answer that, As stated above (Article 5), two things may be considered in unbelief. One of these is its relation to faith: and from this point of view, he who resists the faith after accepting it, sins more grievously against faith, than he who resists it without having accepted it, even as he who fails to fulfil what he has promised, sins more grievously than if he had never promised it. On this way the unbelief of heretics, who confess their belief in the Gospel, and resist that faith by corrupting it, is a more grievous sin than that of the Jews, who have never accepted the Gospel faith. Since, however, they accepted the figure of that faith in the Old Law, which they corrupt by their false interpretations, their unbelief is a more grievous sin than that of the heathens, because the latter have not accepted the Gospel faith in any way at all.
The second thing to be considered in unbelief is the corruption of matters of faith. On this respect, since heathens err on more points than Jews, and these in more points than heretics, the unbelief of heathens is more grievous than the unbelief of the Jews, and that of the Jews than that of the heretics, except in such cases as that of the Manichees, who, in matters of faith, err even more than heathens do.
Of these two gravities the first surpasses the second from the point of view of guilt; since, as stated above (Article 1) unbelief has the character of guilt, from its resisting faith rather than from the mere absence of faith, for the latter as was stated (1) seems rather to bear the character of punishment. Hence, speaking absolutely, the unbelief of heretics is the worst.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)