There are two ideas in this: there is an idea of unity and there is an idea of duality. The idea of unity comes from the realization of God which is the ultimate truth. The idea of duality comes from the absence of this knowledge, and if one has not attained, through the idea of God, that idea of unity, one's realization is not complete. If one has belief in God, but has not arrived at the idea of unity, one has not accomplished the purpose of life.