Objection 1. It would seem that unbelief is not the greatest of sins. For Augustine says (De Bapt. contra Donat. iv, 20): "I should hesitate to decide whether a very wicked Catholic ought to be preferred to a heretic, in whose life one finds nothing reprehensible beyond the fact that he is a heretic." But a heretic is an unbeliever. Therefore we ought not to say absolutely that unbelief is the greatest of sins.
Objection 2. Further, that which diminishes or excuses a sin is not, seemingly, the greatest of sins. Now unbelief excuses or diminishes sin: for the Apostle says (1 Timothy 1:12-13): "I... before was a blasphemer, and a persecutor and contumelious; but I obtained... mercy... because I did it ignorantly in unbelief." Therefore unbelief is not the greatest of sins.
Objection 3. Further, the greater sin deserves the greater punishment, according to Deuteronomy 25:2: "According to the measure of the sin shall the measure also of the stripes be." Now a greater punishment is due to believers than to unbelievers, according to Hebrews 10:29: "How much more, do you think, he deserveth worse punishments, who hath trodden under foot the Son of God, and hath esteemed the blood of the testament unclean, by which he was sanctified?" Therefore unbelief is not the greatest of sins.
On the contrary, Augustine, commenting on John 15:22, "If I had not come, and spoken to them, they would not have sin," says (Tract. lxxxix in Joan.): "Under the general name, He refers to a singularly great sin. For this," viz. infidelity, "is the sin to which all others may be traced." Therefore unbelief is the greatest of sins.
I answer that, Every sin consists formally in aversion from God, as stated above (I-II, 71, 6; I-II, 73, 3). Hence the more a sin severs man from God, the graver it is. Now man is more than ever separated from God by unbelief, because he has not even true knowledge of God: and by false knowledge of God, man does not approach Him, but is severed from Him.
Nor is it possible for one who has a false opinion of God, to know Him in any way at all, because the object of his opinion is not God. Therefore it is clear that the sin of unbelief is greater than any sin that occurs in the perversion of morals. This does not apply to the sins that are opposed to the theological virtues, as we shall stated further on (20, 3; 34, 2, ad 2; 39, 2, ad 3).
Reply to Objection 1. Nothing hinders a sin that is more grave in its genus from being less grave in respect of some circumstances. Hence Augustine hesitated to decide between a bad Catholic, and a heretic not sinning otherwise, because although the heretic's sin is more grave generically, it can be lessened by a circumstance, and conversely the sin of the Catholic can, by some circumstance, be aggravated.
Reply to Objection 2. Unbelief includes both ignorance, as an accessory thereto, and resistance to matters of faith, and in the latter respect it is a most grave sin. On respect, however, of this ignorance, it has a certain reason for excuse, especially when a man sins not from malice, as was the case with the Apostle.
Reply to Objection 3. An unbeliever is more severely punished for his sin of unbelief than another sinner is for any sin whatever, if we consider the kind of sin. But in the case of another sin, e.g. adultery, committed by a believer, and by an unbeliever, the believer, other things being equal, sins more gravely than the unbeliever, both on account of his knowledge of the truth through faith, and on account of the sacraments of faith with which he has been satiated, and which he insults by committing sin.
Source: Summa Theologica (New Advent)