Objection 1. It would seem that hatred of God is not the greatest of sins. For the most grievous sin is the sin against the Holy Ghost, since it cannot be forgiven, according to Matthew 12:32. Now hatred of God is not reckoned among the various kinds of sin against the Holy Ghost, as may be seen from what has been said above (Question 14, Article 2). Therefore hatred of God is not the most grievous sin.
Objection 2. Further, sin consists in withdrawing oneself from God. Now an unbeliever who has not even knowledge of God seems to be further away from Him than a believer, who though he hate God, nevertheless knows Him. Therefore it seems that the sin of unbelief is graver than the sin of hatred against God.
Objection 3. Further, God is an object of hatred, only by reason of those of His effects that are contrary to the will: the chief of which is punishment. But hatred of punishment is not the most grievous sin. Therefore hatred of God is not the most grievous sin.
On the contrary, The best is opposite to the worst, according to the Philosopher (Ethic. viii, 10). But hatred of God is contrary to the love of God, wherein man's best consists. Therefore hatred of God is man's worst sin.
I answer that, The defect in sin consists in its aversion from God, as stated above (Question 10, Article 3): and this aversion would not have the character of guilt, were it not voluntary. Hence the nature of guilt consists in a voluntary aversion from God.
Now this voluntary aversion from God is directly implied in the hatred of God, but in other sins, by participation and indirectly. For just as the will cleaves directly to what it loves, so does it directly shun what it hates. Hence when a man hates God, his will is directly averted from God, whereas in other sins, fornication for instance, a man turns away from God, not directly, but indirectly, in so far, namely, as he desires an inordinate pleasure, to which aversion from God is connected. Now that which is so by itself, always takes precedence of that which is so by another. Wherefore hatred of God is more grievous than other sins.
Reply to Objection 1. According to Gregory (Moral. xxv, 11), "it is one thing not to do good things, end another to hate the giver of good things, even as it is one thing to sin indeliberately, and another to sin deliberately." This implies that to hate God, the giver of all good things, is to sin deliberately, and this is a sin against the Holy Ghost. Hence it is evident that hatred of God is chiefly a sin against the Holy Ghost, in so far as the sin against the Holy Ghost denotes a special kind of sin: and yet it is not reckoned among the kinds of sin against the Holy Ghost, because it is universally found in every kind of that sin.
Reply to Objection 2. Even unbelief is not sinful unless it be voluntary: wherefore the more voluntary it is, the more it is sinful. Now it becomes voluntary by the fact that a man hates the truth that is proposed to him. Wherefore it is evident that unbelief derives its sinfulness from hatred of God, Whose truth is the object of faith; and hence just as a cause is greater than its effect, so hatred of God is a greater sin than unbelief.
Reply to Objection 3. Not everyone who hates his punishment, hates God the author of punishments. For many hate the punishments inflicted on them, and yet they bear them patiently out of reverence for the Divine justice. Wherefore Augustine says (Confess. x) that God commands us to bear with penal evils, not to love them. On the other hand, to break out into hatred of God when He inflicts those punishments, is to hate God's very justice, and that is a most grievous sin. Hence Gregory says (Moral. xxv, 11): "Even as sometimes it is more grievous to love sin than to do it, so is it more wicked to hate justice than, not to have done it."
Source: Summa Theologica (New Advent)