Saint Thomas Aquinas and unbelief

Unbelief is the greatest sin. That Saint Thomas thoughts.
Do you think that that is the so large sin’
I heard from one teological that when is human being atheist that is the most wrong state for that man.

I cannot express myself better than to quote John 3 regarding unbelief being a type of self imposed judgement.

17 For God sent not the Son into the world to judge the world; but that the world should be saved through him. 18 He that believeth on him is not judged: he that believeth not hath been judged already, because he hath not believed on the name of the only begotten Son of God.
John 3:17-18 (ASV)

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