Bhagavad Gita, Chapter 18, Verse 32

That which enveloped in darkness regards Adharma as Dharma and views all things in a perverted light, that intellect, O Pârtha, is Tâmasika.

This intellect is described here as getting things exactly backward, real irreligion mistaken for religion itself.

Krishna names a complete inversion, not a minor error, truth and untruth genuinely swapped in someone's own perception. Real severity marks this lowest kind of intellect.