Oneness of the Divine. It may be given a thousand names such as The Primary Cause / God / Energy / I. All that is created has its Self this Oneness.
One who is full of love attains The states of Oneness with Him.
Whatever books you may read, you cannot realize the Divine merely by intellectual effort. One must put it into practice. That sense of oneness can only be promoted by the practice of love and not by any other means.
It is because the doctrine of human unity based on the spiritual oneness of all beings, is not propagated in the right manner that we have today many divisions leading to many conflicts.