The Quran

وَنَزَعنا مِن كُلِّ أُمَّةٍ شَهيدًا فَقُلنا هاتوا بُرهانَكُم فَعَلِموا أَنَّ الحَقَّ لِلَّهِ وَضَلَّ عَنهُم ما كانوا يَفتَرونَ

Muhammad Asad

And [they will remain silent: for by then] We will have called forth witnesses from within every community,¹ and will have said [unto the sinners]: "Produce an evidence for what you have been claiming!"² And so they will come to understand that all truth is God's [alone];³ and all their false imagery will have forsaken them.⁴

I.e., the prophets who had appeared at various stages of man's history, and who will now bear witness that they had duly conveyed God's message to the people for whom it was meant.
Lit., "Produce your evidence" - i.e., for the possibility of anyone or anything having a share in God's divinity.
I.e., that He is the Ultimate Reality, and that whatever is or could be is an outcome of His will alone.
For the meaning of the phrase ma kanu yaftarun (lit., "all that they were wont to invent" - rendered by me here as well as in 6:24, 7:53, 10:30, 11:21 and 16:87 as "all their false imagery"), see surah 11, note 42; also note 15 on 6:22. A specific instance of such "false imagery" - the futility of man's relying on his own wealth and worldly power - is illustrated in the immediately following legend of Qarun (see next note).