ACIM Quote from Chapter 1, Section 7 - The Illusion of Needs.

The real purpose of this world is to use it to correct your unbelief. You can never control the effects of fear yourself, because you made fear, and you believe in what you made. In attitude, then, though not in content, you resemble your Creator, Who has perfect faith in his creations because he created them. Belief produces the acceptance of existence. That is why you can believe what no one else thinks is true. It is true for you because it was made by you.