You are only human beings like ourselves, and the Most Gracious has revealed nothing. You are only telling lies."
The Messengers said: "Our Lord knows that we have been sent as Messengers to you."
This means that the three Messengers answered them saying:
"Allah knows that we are His Messengers to you. If we were lying, He would have taken the utmost vengeance against us, but He will cause us to prevail and will make us victorious against you, and you will come to know whose will be the happy end in the Hereafter."
This is like the Ayah:
قُلْ كَفَى بِاللَّهِ بَيْنِى وَبَيْنَكُمْ شَهِيداً يَعْلَمُ مَا فِى السَّمَـوَتِ وَالاٌّرْضِ وَالَّذِينَ ءامَنُواْ بِالْبَـطِلِ وَكَفَرُواْ بِاللَّهِ أُوْلَـيِكَ هُمُ الْخَـسِرُونَ
Say:
"Sufficient is Allah for a witness between me and you. He knows what is in the heavens and on earth."
And those who believe in falsehood, and disbelieve in Allah, it is they who are the losers. (29:52)
وَمَا عَلَيْنَا إِلاَّ الْبَلَغُ الْمُبِينُ