Verily We have guided him to the way We have pointed out to him the path of guidance by sending messengers to mankind whether he be grateful that is to say whether he be a believer or ungrateful both shākiran and kafūran are circumstantial qualifiers referring to the direct object; in other words We have pointed out to him in both presupposed states whether his gratefulness or his unthankfulness; immā is used to list the ‘states’.
