Quote from ACIM Text Chapter 1, Section 5 - The Escape from Darkness.

Holiness can never be really hidden in darkness, but you can deceive yourself about it. This deception makes you fearful because you realize in your heart it is a deception, and you exert enormous efforts to establish its reality. The miracle sets reality where it belongs. Reality belongs only to spirit, and the miracle acknowledges only truth. It thus dispels illusions about yourself, and puts you in communion with yourself and God. The miracle joins in the Atonement by placing the mind in the service of the Holy Spirit. This establishes the proper function of the mind and corrects its errors, which are merely lacks of love. Your mind can be possessed by illusions, but spirit is eternally free. If a mind perceives without love, it perceives an empty shell and is unaware of the spirit within. But the Atonement restores spirit to its proper place. The mind that serves spirit is invulnerable.