Gregory the Dialogist

Which is the chiefest iniquity, and denial against the Most High God. For it is clear that he does deny Him, when setting at nought His grace, he claims to himself the powers of good practice. Which too is rightly called as well 'the chiefest iniquity' because every act of sin which is from infirmity destroyeth not hope, seeing that it asks forgiveness from the Judge Above. But presuming on our own goodness is so much the worse in desperateness, the further it is removed from humility. …

Morals on the Book of Job, Book XXII ↗

Thomas Aquinas

He shows why he avoided this saying, "this is the greatest evil." For if it is evil to offer what is due to one man to another, it seems the greatest evil that the cult due to God is offered to a creature. Since it is impossible for man to offer divine worship at the same time to God and a creature, he then says, "and the denial of God the Most High." Even though the name of God is attributed to some creatures by participation, the cult of latria is due only to the most high God, who is denied to be the most high if that cult is also …

Commentary on Job ↗