In my experience, seeing the common in all is crucial. It can only be done by withdrawing one’s attention from the external identity (I.e., race, country, culture, etc) and rooting it in the inner. Self knowledge, this way, leads to the realization that we are all one. But this rooting attention on one’s true nature and away from external identity must become a habit. Only then can we become truly aware of the oneness of all without having to rely on belief or dogma.