An illusion is one's own interpretation and perception of someone or something. It can be a strong belief or a wish. They are not necessarily false or errors.
The strength of the illusion lies in the strength of wish fulfillment. For example - People, at first, interpret phenomena's according to their experience and knowledge of their every day life. They project their own character into the circumstance presented and regard it as somehow "ensouled". Such indefinite notions are transformed into a distinct conception of a higher order than human beings, and yet somehow resembling them. As it may seem, humans have a metaphysical (speculative) need for a Supreme Being. God, as we call him. He is the eternal and infinite Spirit, Creator of the Universe, and the ultimate power. We present him with human characteristics in all concepts of God, which has led me to believe that it is not God who created man in His image, but man who creates God in his.
Religion is one such need based on myth and spiritualism. People are taught about their religion at a young age, and when they grow up they attempt to account for things using the notion of their religion as the basis for their speculations.
On the other hand, there is no truth so certain as the existence of God. He is the groundwork of all our hopes, and our foundation of morality and society. Nothing exists without a cause. Although we can't demonstrate a soul-substance and the immortality of a soul, or demonstrate anything concerning the nature of God, there is still a dependency on him, so that we cannot ascribe a personality to God, in the case that it would make him finite.
God is the universe conceived as an eternal and necessary unity.
Benedict Spinoza - rationalist
Freidrick Schleiermacher - religion
Sigmund Freud - atheist