Why is 'they will come to know' repeated twice in Surah An-Naba?
Answer
The repetition of 'Kallā sayaʿlamūn' in 78:4 and 'Thumma kallā sayaʿlamūn' in 78:5 is a deliberate rhetorical device the Arabic language uses for emphasis (taʾkīd) and intensification. Classical commentators such as al-Saʿdī and Ibn Kathīr state that the second verse repeats the first to reinforce and double the threat against those who denied the resurrection, so they could not treat it as a passing remark. The added word 'Thumma' ('then,' 'again') does not introduce a new event but layers a second wave of warning on top of the first, conveying that their reckoning is certain and that the realization awaiting them is overwhelming. This pattern of escalating repetition is characteristic of Qur'anic warnings to the heedless. Some exegetes, recorded by al-Ṭabarī and al-Qurṭubī, suggested the two phrases address different audiences — the first warning disbelievers, the second relating to the believers' outcome — but the reading conveyed as stronger is that both verses warn the same deniers with mounting force. The effect on the listener is psychological: the unresolved, unnamed object of 'they will know' grows heavier with each repetition, building dread before the surah goes on to describe God's signs and the Day of Decision.
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In more depth
This emphatic doubling sets up the surah's pivot: from the dispute over the resurrection (78:1-5) the discourse turns to the signs of God's power in creation (78:6-16) and then to the Day of Judgment itself (78:17 onward), so the warning is grounded in evidence rather than left as a bare threat.