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Lit., "in the decree" - i.e., in accordance with the immutable laws which God has laid down for His creation.
I.e., since everything in this world is ephemeral and bound to perish, man ought to be conscious of the life to come.
These verses are a commentary on the last clause of the last verse. "The Wrath of thy Lord is something to take heed of." The godless thoughtlessly challenge Allah's Wrath, but do they realise its nature? Even the best of us must be moved with terror when we think of its consequences, were it not for His unbounded Mercy. Those who deny the Hereafter fail to realise its terrible Portents. They ask for Portents and Miracles now, but do they realise that their coming means destruction and misery to those who reject faith? They will come soon enough. The whole world will be convulsed before the Day of Judgment. The part of the wise is to prepare for it.