sufficiency of Allah's support

وَيَقُولُ ٱلَّذِينَ كَفَرُوا۟ لَسْتَ مُرْسَلًا ۚ قُلْ كَفَىٰ بِٱللَّهِ شَهِيدًۢا بَيْنِى وَبَيْنَكُمْ وَمَنْ عِندَهُۥ عِلْمُ ٱلْكِتَـٰبِ

And those who have disbelieved say, "You are not a messenger." Say, [O Muḥammad], "Sufficient is Allāh as Witness between me and you, and [the witness of] whoever has knowledge of the Scripture."