(17–18) “How, then, if you disbelieve, will you protect yourselves from a Day that will turn children grey-haired? * The heaven will be rent asunder by it. His promise is bound to be fulfilled.”

That is: How will you gain deliverance and salvation from the Day of Resurrection—a Day whose events are terrifying and whose magnitude is immense (1), which will turn children grey-haired and cause even the mightiest solid masses to melt? Because of it, the heaven will split apart and its stars will scatter. “His promise is bound to be fulfilled”—that is, it must inevitably occur, and nothing can prevent it.