Avraham Avinu is instructed by Hakadosh Baruch Hu to go up to the mountain and to bring Yitzchok as a Korban. R' Moishe Rosenstein gives a very beautiful and powerful insight as to why this was such a difficult test.
He says, that when someone has a test and it's in front of everyone, then as difficult as the test may be it's still much easier to pass the test. And the reason is because you are in the public eye. But the true test of character is when no one else is around.
Avraham Avinu went up to the mountain, no one else was there besides him and Yitzchok. He theoretically could have said to Yitzchok, alright I don't want to kill you. I will send you down the other side of the mountain and you will be fine. And no one will ever know the difference. And that was the nisayon.
There are many times when we are given nisyonos, challenges and no else is around. If you want to really know what you are made of, that is the moment. It is only you and Hakadosh Baruch Hu . What will you do?