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Namely, that their preference of worldly benefits to a spiritual commitment is and, therefore, morally "justified". It is to this flagrant self-deception that the next sentence refers.
The definite article al prefixed to the participial noun kadhibun indicates that the people thus characterized have reached the utmost degree of self-deception; hence my interpolation of the adjective "greatest" in consonance with Zamakhshari's interpretation of the above phrase.
When Judgment is established, and before they the Truth, they may think (as now) that some oaths or excuses will save them. But they have not now-much less will they have then-any footing to stand upon. Falsehood is falsehood, and must perish. They must learn the worthlessness of their falsehood.