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This connects with the first clause of verse {17} above, which, in its turn, contains an allusion to the mentality spoken of in verses {14-15}.
i.e., if only the Meccan pagans knew.
Even in this life the punishment for heedless or selfish arrogance and sin comes suddenly when we least expect it. But there is always room for Allah's Mercy if we sincerely repent. If the Punishment in this life seems to us so stupefying, how much worse will it be in the Hereafter, when the Punishment will not be only for a limited time, and the time for repentance will have passed?