Thomas Aquinas

“ Hence whatever befits God, pertains to His goodness, and whatever does not befit Him, is far removed from the perfection of goodness which is His Essence. Consequently whoever either denies anything befitting God, or affirms anything unbefitting Him, disparages the Divine goodness.
Now this may happen in two ways. In the first way it may happen merely in respect of the opinion in the intellect; in the second way this opinion is united to a certain detestation in the affections, even as, on the other hand, faith in God is perfected by love of Him. ”

“ Now the sin of murder, since it destroys a man's life, does more harm than the sin of blasphemy, which can do no harm to God. Therefore the sin of murder is more grievous than that of blasphemy.
Objection 2: Further, a perjurer calls upon God to witness to a falsehood, and thus seems to assert that God is false. But not every blasphemer goes so far as to say that God is false. Therefore perjury is a more grievous sin than blasphemy.
Objection 3: Further, on Ps. 74:6, "Lift not up your horn on high," a gloss says: "To excuse oneself for sin is the greatest sin of all." ”

“ Since, as stated above (A [1] ) , blasphemy is contrary to the confession of faith, its prohibition is comprised under the prohibition of unbelief, expressed by the words: "I am the Lord thy God," etc. (Ex. 20:1) . Or else, it is forbidden by the words: "Thou shalt not take the name of . . . God in vain" (Ex. 20:7) . Because he who asserts something false about God, takes His name in vain even more than he who uses the name of God in confirmation of a falsehood. ”