So that He may know they have conveyed the Messages of their Lord, and He may encompass whatever is with them, and He has inscribed the number of everything.
That means the Three Time Knower Allah who is acquainted with everything has already inscribed everything that will happen in the universe in His Book Adhkr, the preservation which is undertaken by Him. It is according to that Book He decides whom, when and where Messengers are to be deputed, and who will answer to the invitation of Messengers positively and negatively. “Surely He may know that they have conveyed the Messages of their Lord” means to know for the believers who bears vicegerent of Allah. By understanding this fact, the believer by utilizing Adhikr-the Confirming Truth-will make sure that his fate is to the Paradise, and will propagate Adhikr throughout the world aiming to extend the lifetime of universe. See explanation 19: 64; 65: 12; and 69: 51