Atheism is the acceptance of the following assumption:

Theism cannot be definitively and empirically proven and is therefore untrue.

This assumption leads to the rejection of religion when individual atheist decide that they are unconvinced by the available data. Acceptance of this assumption is an error for the following reasons:

1) Metaphysically because the choice (in isolation) voids the existing moral structure rendering the decision itself morally incoherent.

2) Biologically because a sound moral structure is necessary for a healthy life and rejecting traditional structures appears to reduce health, happiness, and fertility.

3) Anthropologically because religion is a critical and perhaps primary mechanism for overcoming our species-specific upper limit to group size which is set by purely cognitive constraints.

In addition we must consider that the assumption itself may simply be false. Specifically:

Theism may be correct and true even if it cannot be definitively and empirically proven.

Atheism as opposed to agnosticism is a statement of certainty. True belief in an unproven assertion is an act of faith. I will leave it to the reader to decide if this specific act of faith constitutes a religion.