"....and when we realize this fully we realize that it is not One but Zero"
When a reference is made to this "zero" or "emptiness, the prime significance is that we are at the last unable to define it, 'It' is neither One nor is 'It' zero, but strangely in a sense both, but that is not quite correct either.
But the essence of it is to cease defining anything ultimately (but it is of course ok relatively, for living purposes), to cease mental comparisons and categories, and allow it simply to be or not be.
This existence is "one" in the sense that all is linked and interdependent on all else, it is empty or zero in the sense that what 'It' is cannot be grasped or bounded, and in the sense that for instance, light cannot be defined without darkness. One must simply let go and float with this Thusness, allow this "Emptiness" to speak of its own way and time. It contains all, even the concept of mystery...It is a wonderful infinitely unfolding Mystery and "God" will ever be a mystery unto "Himself" (in theistic personification).
By the way, it is not necessary to spend long years in meditation in a cave or become a person of unstainable purity or spiritual magnificence to see this. It is available to all at every moment. Our "self" is a construct, that like a body, is required to exist in this manifested way. But ultimately we too are this Emptiness.
Clinging to things like a "self" or a "doctrine" or "a cause" gets in the way, even though all these things are natural and at times helpful in human life.
Life is there each day, out of whatever "empty" event occurs to this "empty" self, let the drama unfold and the challenges and joys be met and happiness obtained.
Forgive my poor words, they are but a stumbling pointing.