How do we know that the Prophets of Allah do not falter in delivering the Message of the Almighty?
The Messengers of Allah are protected and guarded by Allah from faltering in delivering the divine message. In their capacities as divine appointees, they present to their addressees the very message of Allah without any adulteration and alteration. It is the Almighty Himself who takes care of this:
As for those whom He chooses as His messenger [they can say nothing of their own] because He guards them from behind and from the front that He may know if they have delivered the messages of their Lord; and He surrounds whatever is around them and keeps count of all things. (72:16-28)
The Almighty also protects His Rusul from being harmed by their people when they are delivering the truth to them:
Before them the People of Noah denied and the confederates after them; and every people tried to seize their Rasūl and disputed by means of vanities, therewith to condemn the Truth: but it was I Who seized them! And how terrible was My requital! (40:5)