The magnitude of the punishment matches the magnitude of the sin. Now a sin that is against God is infinite; the higher the person against whom it is committed, the graver the sin-it is more criminal to strike a head of state than a private citizen-and God is of infinite greatness. Therefore an infinite punishment is deserved for a sin committed against Him.

Summa Theologiae: Volume 27, Effects of Sin, Stain and Guilt: 1a2ae. 86-89 (ed. Cambridge University Press, 2006) - ISBN: 9780521029353