Resurrection
They swear by Allah with their strongest oaths that He will not resurrect the dead. But yes, this is a true promise upon Him, though most people do not know.
16:38And they swear by Allah their most binding oaths (that) Allah will not raise up him who dieth. Nay, but it is a promise (binding) upon Him in truth, but most of mankind know not,
To clarify what they differed about, and so the disbelievers may realize they were wrong.
16:39That He may explain unto them that wherein they differ, and that those who disbelieved may know that they were liars.