Objection 1. It would seem that schism is a graver sin than unbelief. For the graver sin meets with a graver punishment, according to Deuteronomy 25:2: "According to the measure of the sin shall the measure also of the stripes be." Now we find the sin of schism punished more severely than even the sin of unbelief or idolatry: for we read (Exodus 32:28) that some were slain by the swords of their fellow men on account of idolatry: whereas of the sin of schism we read (Numbers 16:30): "If the Lord do a new thing, and the earth opening her mouth swallow them down, and all things that belong to them, and they go down alive into hell, you shall know that they have blasphemed the Lord God." Moreover the ten tribes who were guilty of schism in revolting from the rule of David were most severely punished (2 Kings 17). Therefore the sin of schism is graver than the sin of unbelief.
Objection 2. Further, "The good of the multitude is greater and more godlike than the good of the individual," as the Philosopher states (Ethic. i, 2). Now schism is opposed to the good of the multitude, namely, ecclesiastical unity, whereas unbelief is contrary to the particular good of one man, namely the faith of an individual. Therefore it seems that schism is a graver sin than unbelief.
Objection 3. Further, a greater good is opposed to a greater evil, according to the Philosopher (Ethic. viii, 10). Now schism is opposed to charity, which is a greater virtue than faith to which unbelief is opposed, as shown above (10, 2; 23, 6). Therefore schism is a graver sin than unbelief.
On the contrary, That which results from an addition to something else surpasses that thing either in good or in evil. Now heresy results from something being added to schism, for it adds corrupt doctrine, as Jerome declares in the passage quoted above (1, ad 3). Therefore schism is a less grievous sin than unbelief.
I answer that, The gravity of a sin can be considered in two ways: first, according to the species of that sin, secondly, according to its circumstances. And since particular circumstances are infinite in number, so too they can be varied in an infinite number of ways: wherefore if one were to ask in general which of two sins is the graver, the question must be understood to refer to the gravity derived from the sin's genus. Now the genus or species of a sin is taken from its object, as shown above (I-II, 72, 1; I-II, 73, 3). Wherefore the sin which is opposed to the greater good is, in respect of its genus, more grievous, for instance a sin committed against God is graver than a sin committed against one's neighbor.
Now it is evident that unbelief is a sin committed against God Himself, according as He is Himself the First Truth, on which faith is founded; whereas schism is opposed to ecclesiastical unity, which is a participated good, and a lesser good than God Himself. Wherefore it is manifest that the sin of unbelief is generically more grievous than the sin of schism, although it may happen that a particular schismatic sins more grievously than a particular unbeliever, either because his contempt is greater, or because his sin is a source of greater danger, or for some similar reason.
Reply to Objection 1. It had already been declared to that people by the law which they had received that there was one God, and that no other God was to be worshipped by them; and the same had been confirmed among them by many kinds of signs. Consequently there was no need for those who sinned against this faith by falling into idolatry, to be punished in an unwonted manner: it was enough that they should be punished in the usual way. On the other hand, it was not so well known among them that Moses was always to be their ruler, and so it behooved those who rebelled against his authority to be punished in a miraculous and unwonted manner.
We may also reply by saying that the sin of schism was sometimes more severely punished in that people, because they were inclined to seditions and schisms. For it is written (Ezra 4:15): "This city since days gone by has rebelled against its kings: and seditions and wars were raised therein [Vulgate: 'This city is a rebellious city, and hurtful to the kings and provinces, and... wars were raised therein of old']." Now sometimes a more severe punishment is inflicted for an habitual sin (as stated above, I-II, 105, 2, ad 9), because punishments are medicines intended to keep man away from sin: so that where there is greater proneness to sin, a more severe punishment ought to be inflicted. As regards the ten tribes, they were punished not only for the sin of schism, but also for that of idolatry as stated in the passage quoted.
Reply to Objection 2. Just as the good of the multitude is greater than the good of a unit in that multitude, so is it less than the extrinsic good to which that multitude is directed, even as the good of a rank in the army is less than the good of the commander-in-chief. On like manner the good of ecclesiastical unity, to which schism is opposed, is less than the good of Divine truth, to which unbelief is opposed.
Reply to Objection 3. Charity has two objects; one is its principal object and is the Divine goodness, the other is its secondary object and is our neighbor's good. Now schism and other sins against our neighbor, are opposed to charity in respect of its secondary good, which is less than the object of faith, for this is God Himself; and so these sins are less grievous than unbelief. On the other hand, hatred of God, which is opposed to charity in respect of its principal object, is not less grievous than unbelief. Nevertheless of all sins committed by man against his neighbor, the sin of schism would seem to be the greatest, because it is opposed to the spiritual good of the multitude.
Source: Summa Theologica (New Advent)