To be a true theist you have to have proof positive that there is a God. Because no theist possesses that ability they are all make believers and teller of tall tales.
Further, as most act and live their lives not in accordance to the very rules and tenets of their alleged faith, it is obvious that they have no belief in diety. Hypocrisy is the only law by which they abide.
Atheists, on the other hand, tend to have something of an axe to grind, a grievance that reeks far more intimate. Their approach to and investment in the subject ironically smacks of someone who IS a believer in a GodS, albeit antagonistically.
Like my Ol' Granpappy Modda used to say, "Nobody believes in God like an Atheist, because with them it's personal,,,"