The Quran

وَإِذا أَذَقنَا النّاسَ رَحمَةً مِن بَعدِ ضَرّاءَ مَسَّتهُم إِذا لَهُم مَكرٌ في آياتِنا ۚ قُلِ اللَّهُ أَسرَعُ مَكرًا ۚ إِنَّ رُسُلَنا يَكتُبونَ ما تَمكُرونَ

Muhammad Asad

And [thus it is:] whenever We let [such] people¹ taste [some of Our] grace after hardship hasvisited them-lo! they forthwith turn to devising false arguments against Our messages.² Say: "God is swifter [than you] in His deep devising!" Behold, Our [heavenly] messengers are recording all that you may devise!

I.e., the two categories of people referred to in verses 7, 11, 12, 15, 18 and 20.
Lit., "they have forthwith a scheme against Our messages". (The particle idha preceding this clause is meant to bring out the element of immediacy, and is best rendered as "lo! they forthwith...", etc.) Since God's messages are purely conceptual, the "scheming against them" obviously connotes the devising of fallacious arguments meant to cast doubt on the divine origin of these messages or to "disprove" the statements made in them. The above discourse on the psychology of agnosticism and half-belief is continued in the parable of the seafarers set forth in the next two verses.