Why 'should' atheists be moral?

[Mystic]

In other words, an all-encompassing sense that originates in and by your conceptions of life and purpose.

I'm still not sure what you mean, but I'm going to say we have a sense that ultimate value exists without fully perceiving the full extent of that being as it is beyond us. The all-value is belief that all value has a basis in the divine. When we value something it's usually indirectly valuing life or a state of being, and this life and state of being has it's essence being passed on from the Divine. The creator being thte source and basis to value can be see as one of the divine names or instance of the name of God. God is outward and the inward. The outward is descent of his essence to the world of creation and the inward is the origin and the manifested by the outward. He is inward in his outward and outward in his inward.

We don't need to have a full comprehension of God to have some idea of God. God is unknown and unseen but at the same time is manifest and close.