What is the ruling regarding the sect that denies Divine Decree (Qadar)?

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The Prophet (peace be upon him) stated that every nation has its Magians (fire-worshippers), and the Magians of this Ummah are those who say there is no Divine Decree (La Qadar). Therefore, they are not to be visited if they fall ill, and their funerals are not to be attended if they die, as narrated by Ibn Umar and recorded by Imam Ahmad.