Ayn Rand and Atheism

How can Objectivism accept atheism - the nonexistence of god(s), when atheism presupposes that all knowledge is known?

Objectivism accepts that knowledge is gained by applying reason and logic to sensory data. What sensory data supports the nonexistence of god(s)? I assert that denying the existence of a deity is illogical because it is dependent on the false premise that all knowledge is known. By what process of non-contradictory identification does one take to deny for certain the existence of a deity?

Atheism itself is a contradiction. To believe fully in the nonexistence of something without knowing all knowledge is to take it on faith. If by definition agnosticism rejects the unknown (not the unknowable), I assert Rand would have been better suited to take an agnostic stance with respect to the existence of God.