(the Most Merciful.) means, towards His believing servants.

((It is) a promise of Allah, and Allah fails not in His promise.) means, ‘this that We have told you, O Muhammad, that the Romans will defeat the Persians, is a true promise from Allah, truth which cannot be changed and which will surely come to pass, because Allah has decreed that the one of the two parties that is closer to the truth will prevail and that victory will be theirs.’

(but most men know not) means, they do not know the decree of Allah concerning His creation, but what He does is wise and is based on justice. His saying: