You commence your journey from the stage of dualism and finally end up in the stage of non-dualism. You start your spiritual practice with the very ordinary type of devotion, worshipping God with form and attributes and using rituals and external forms of worship. But then you quickly progress onto the formless, the absolute aspect of the Divinity. In this way, you initially develop yourself spiritually by being a servant of God, but eventually you become fully identified with God.
Consider, for a moment, a very big circle, and consider that just by its side and separate from it there is another circle, one which is very much smaller. The big circle may be thought of as God, the small one as the individual soul. Here the individual is different and distinct from God; this is dualism. When you bring the smaller circle in so that it lies within the bigger circle, you have qualified non-dualism; now the individual is part of the Divinity, he exists in God. What then is the meaning of the individual becoming totally merged in God? The small circle has to broaden itself and grow bigger and bigger until it has fully expanded to the size of the big circle. At that point the two circles are indistinguishable and man has merged himself into God. This is complete non-dualism.