One of the basic teachings of the Torah is that God does not expect of a human being anything which is beyond the human capacity to carry out. This is quite understandable, for even a human being, who is very far from absolute perfection, would not expect of a tool that he has fashioned any more than he has put into it.
Certainly God, the Creator of man, knows man's capacities. From this, it immediately follows that when a person faces any kind of a test of faith, it is certain that he has been given the capacity to overcome it. And the more difficult the test, the greater are the individual's capacities.
The reason that an individual is tested is not because God wants to know how he will acquit himself, but in order that this person be afforded the opportunity to realize his potential, even that which is unknown to him.
And when one's potential capacities are released and activated, they become part and parcel of his or her arsenal, to be used for personal as well as communal benefit.
Source: Excerpt from a letter of the Lubavitcher Rebbe