The unknown god is your own human imagination, worshipped without being recognized. People build their altars to luck, to outer powers, to a deity somewhere beyond them, and all the while the thing they are truly serving is the power to imagine, unnamed and unseen at the center of their own being. That is the god behind every altar, honored blindly, credited to everything except itself.
To declare this unknown god is simply to name it: the creator you have been reaching toward is the imagination within you, and nothing outside is the cause. Every idol and object a person credits is really this one power at work, mistaken for something external. When a person feels a state as though it were so, his imagining makes it real, because imagination is God. The god men have set up altars to without knowing his name is the awareness in which they live, and it becomes whatever they assume.