Human beings want things (to stay alive, to eat ice cream, etc.).  Rational secular morality is the game-theory-based mutual respect we have for each other's wants.  The purpose is for each individual to maximize the expected value of the net satisfaction of their wants.  Any time we don't respect peoples desires, there is an evil aspect to it.  In many cases, like with the desire to commit crimes, the desire itself represents a greater disrespect for the wants of others.  So there is a net benefit to not respecting it.  Many people want to hold benign theist beliefs.  If you don't respect that, there is an evil aspect to doing so.  The offseting benefit, preventing the lending of creedence to malignent theist beliefs, seems very weak.  It's a logical possibitlity, but a practical case seems very implausible.  Osama Bin Laden would not have a crisis of belief in his extra-psycho Wahabbism if Methodism suddenly disappeared.  So there is no serious offsetting benefit to the evil of not respecting the yearning for God.  Thus, anti-theism is a net evil under rational secular morality.