Unity comes first and last, In between we have the illusion of duality, which we believe for a long time.  

This apparent duality is ultimately shown to be an illusion, but it's an illusion through which we must pass through which we learn and grow and then once again return to Unity

You may have had mystical experiences where you have had true union with other beings or even with the unity, THE ONE WHO'S ALL , such experiences change our lives, effectively showing us that the saying, We are  all one, is just not a pious belief, but a statement of actual fact.