{ 1} It means that their arguments against Islam having failed on rational grounds, the only possibility was that they had a direct link with Allah whereby they have ensured that their faith was true, but no one could bring any proof of having such a link with Allah. (Muhammad Taqi Usmani)
{ 2} This points out to the demand of the pagans of Makkah that they would not believe in the Holy Prophet unless he brings some pieces of the sky falling down from above. (See 17:92 and 26:187 in the Qur an) The answer given is that even if Allah fulfils this absurd demand, they would still not believe and would seek other excuses like claiming that the mass they are seeing is nothing but a cloud. (Muhammad Taqi Usmani)