וַיֹּ֨אמֶר דָּוִ֜ד לַאֲחִימֶ֣לֶךְ הַכֹּהֵ֗ן הַמֶּ֘לֶךְ֮ צִוַּ֣נִי דָבָר֒ וַיֹּ֣אמֶר אֵלַ֗י אִ֣ישׁ אַל־יֵ֧דַע מְא֛וּמָה אֶת־הַדָּבָ֛ר אֲשֶׁר־אָנֹכִ֥י שֹׁלֵחֲךָ֖ וַאֲשֶׁ֣ר צִוִּיתִ֑ךָ...
David answered the priest Ahimelech, “The king has ordered me on a mission, and he said to me, ‘Absolutely no one must know anything about the mission on which I am sending you and for which I have given you orders.’…
(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation. Before accounting for this rendering, I will analyze the plain sense of the Hebrew term containing אִישׁ, by employing a situation-oriented construal as outlined in this document, pp. 11–16.)
The situating noun אִישׁ is commonly used in schematically framed prohibitions such as this one. The fronting of אִישׁ before the negating word is a marked syntax (“constituent focus”); it indicates that exceptions are not tolerable. With the negator אַל, such fronting occurs only twice more with this verb (Jer 36:19; 38:24) and at least 7 times with other verbs—whereas the unmarked word order is less frequently attested (Exod 16:29; Hos 4:4, 2nd instance). Compare my comment at the previous verse; and see further my comment at Judg 21:1 (2 of 2).
As for rendering into English, the NJPS ‘No one must know’ lacks the stronger implication that is generated by the marked word order. The revised rendering expresses the emphatic nuance in spoken idiom.