قُلْ كَفَى بِاللَّهِ بَيْنِي وَبَيْنَكُمْ شَهِيدًا
...
Say: "Sufficient is Allah for a witness between me and you..."
`He knows best the words of denial that you utter, and he knows what I am telling you about Him and that He has sent me. If I were telling lies about Him, He would have executed His vengeance upon me,'
as Allah says elsewhere:
وَلَوْ تَقَوَّلَ عَلَيْنَا بَعْضَ الاٌّقَاوِيلِ
لاأَخَذْنَا مِنْهُ بِالْيَمِينِ
ثُمَّ لَقَطَعْنَا مِنْهُ الْوَتِينَ
فَمَا مِنكُم مِّنْ أَحَدٍ عَنْهُ حَـجِزِينَ
And if he had forged a false saying concerning Us, We surely would have seized him by his right hand, and then We certainly would have cut off his aorta, and none of you could have withheld Us from (punishing) him. (69:44-47)
`But I am telling the truth in what I say to you about Him, so He has supported me with clear miracles and definitive evidence.'
...
يَعْلَمُ مَا فِي السَّمَاوَاتِ وَالاَْرْضِ
...
He knows what is in the heavens and the earth.
means, nothing is hidden from Him at all.
...
وَالَّذِينَ امَنُوا بِالْبَاطِلِ وَكَفَرُوا بِاللَّهِ أُوْلَيِكَ هُمُ الْخَاسِرُونَ
And those who believe in falsehood, and disbelieve in Allah, it is they who are the losers.
means, on the Day of Resurrection, they will be punished for what they did, and will get what they justly deserve for rejecting the truth and following falsehood, for disbelieving in the Messengers of Allah even when there was proof that they were telling the truth, and for worshipping false gods with no evidence.
Allah will punish them for all that, for He is All-Wise and All-Knowing.