Abdu’l Baha taught that although compared to God the world of becoming can be seen as an illusion it clearly possesses a relative reality.
“Certain sophists think that existence is an illusion, that each being is an absolute illusion which has no existence—in other words, that the existence of beings is like a mirage, or like the reflection of an image in water or in a mirror, which is only an appearance having in itself no principle, foundation or reality. This theory is erroneous; for though the existence of beings in relation to the existence of God is an illusion, nevertheless, in the condition of being it has a real and certain existence. It is futile to deny this. For example, the existence of the mineral in comparison with that of man is nonexistence, for when man is apparently annihilated, his body becomes mineral; but the mineral has existence in the mineral world. Therefore, it is evident that earth, in relation to the existence of man, is nonexistent, and its existence is illusory; but in relation to the mineral it exists.In the same manner the existence of beings in comparison with the existence of God is but illusion and nothingness; it is an appearance, like the image reflected in a mirror. But though an image which is seen in a mirror is an illusion, the source and the reality of that illusory image is the person reflected, whose face appears in the mirror. Briefly, the reflection in relation to the person reflected is an illusion. Then it is evident that although beings in relation to the existence of God have no existence, but are like the mirage or the reflections in the mirror, yet in their own degree they exist”.