We are talking about duality within the duality. The duality exists until the mind exists. Absent of mind is absent of duality. Therefore, by understanding "What is mind?" and "What is the substance of the mind?" one becomes aware what is reality, and what is illusion. Only by perfect understanding and realization of the nondual truth, one will be able to overcome the duality.
Since we are conditioned to view, judge and conclude the truth on the base of `I', which is the false self, within the false experience, we are unaware what is truth, and what is illusion. Man and his world exist within the illusion. Therefore, it is erroneous to view and judge the truth on the base of illusory entity.
Truth cannot be got in world. The truth is hidden within the three states as their formless substance and witness. The formless substance is the soul. And the soul is the true self.
The true self /soul is not an entity, or identity within the waking /dream, but it is the source of the three states, from where the three states erupt and subsides.
When one is able to view and judge the worldview on the standpoint soul as self, then he will realize there is neither the creation, nor the creator, but only non-dual soul /spirit will prevail as ultimate reality. Therefore, it is erroneous to conclude the truth on the base of religion and god.
All religious and yogic practices are based on the `I' as self, which is false identity within the false experience [waking/duality] . The Waking experience is mere illusion, same way as the dream. Hence it is very much necessary to verify truth of the `I', to realize the fact that `I' itself is mere illusion. `I' is not the self, because it comes and goes as waking/dream. Absent of `I' absent of waking/dream/ mind. Absent of waking/dream is absent of illusion/duality. The knower of the `I' is formless soul. When one who realizes the soul as the true self, and then he becomes free from experiencing the duality as reality.
This formless path is very difficult for those who stick to their inherited or accepted truth, but not for the serious seekers who are in search of only non dual truth nothing but non dual truth.