Perhaps the most popular condemnation of religious faith of any sort is the inhumane acts of violence committed in the name of one deity or another. Indeed, history is replete with examples of breathtaking atrocities in the name of God or Allah or any other collective title for the many deified ideas worshiped by one group or another. The most immediate examples vilify Islam; but the pedophilic Catholic hierarchy makes many of the Christian community blush. Even Protestant Christianity has its own set of crosses to bear, having long-perpetuated cultures of anti-Semitism and racial- and gender-discrimination. In light of the many moral failures of proponents of the plethora of religious organizations, non-belief seems valid, and faith can seem downright foolish.
But dismissing a philosophy because of the shortcomings of its adherents is naïve at best. In evaluating a philosophy, one must consider it theoretically. Consider a doctrine based on its hypothetical principles completely fulfilled, not on the failure of any individual(s) to adhere to its tenets. It is a devilishly brilliant scheme to dismiss an unpleasant or undesired dogma on account of a poor representation of it in practice, but it is intellectually lazy and dishonest. Validity of dogma does not rely on the adherents. A dogma is true or false on its own merit.
How relieving is that? Speaking as an utter failure as a Christian, I'm sustained by knowing that God's existence and truth is not determined by my ability to live according to Christian principles. If it were, if the Christian doctrine required adherents to be perfect in order to be valid, then who would need the Christian doctrine? Unfortunately for critics, Truth is Truth, regardless and independent of humanity.
And that is Great.