How do you know the state of our Essential Nature that you experience is not a passing state?
Because the very nature of the Essential State engenders the effortless conviction that there is no I who experiences or can experience it. And because it is Not Two there is no opposite to it. Therefore there is no passing any more than there is a coming.
What comes and goes and changes for the seeker is the intensity of the sense of separation. After Spiritual Realization those cycles no longer take place.