There's an interesting discussion going on over at the "Cosmic Variance" blog.
As a Christian I am not of those who think atheists are persons of low or no integrity.
On the contrary I am certain that most are persons of high integrity.
Just remember, however, that it is much easier for someone whose belief system is defined in terms of negation (as in "a-" theist or "non-" Christian) to maintain a position whereby any lack of integrity is less perceptible or even completely imperceptible: What is their standard? Who sets it? And who judges whether they have achieved it?
Christians, on the other hand, hold to a positive personal ethic which is externally and not subjectively determined. So they are at a clear disadvantage. Any shortcomings are glaring and so open to being boldly trumpeted and paraded everywhere by the uncharitable.
Also note that while very bad things have been and continue to be done in the name of God ( John16:2 ) or of religion the Guinness World Record holders of evil are the atheists like Stalin, Mao, and Pol Pot.
Christians aren’t perfect just forgiven.