They're not true Christians
"They're not true Christians," is a phrase oft-uttered among Christian apologists to "disown" examples of self-proclaimed Christians who have committed acts widely condemned as immoral. Frequently, the same Christians who use this phrase will also criticize atheism for lack of moral oversight, often even going so far as to point out immoral atheists or supposed atheists in an effort to extrapolate to atheism as a whole -- a form of flawed inductive reasoning. Furthermore, it may be argued that they are committing the fallacy of special pleading, wherein apologists hold themselves to a different standard than those with whom they disagree, as they define "true Christians" as those who never commit immoral acts. Often, such apologists ignore the large number of atheists with outspoken dedication to moral principles, either informally or through adherence to schools of thought such as secular humanism. Clearly, such a statement merely serves to whitewash Christianity as a bastion of morality, while juxtaposing the supposed dangers of atheism and the atrocities it may bring.
More pragmatic examination of this argument yields further weaknesses. First, one may ask, where is the line between "true Christian" and "not a true Christian"? Clearly, some immoral acts are far worse than others, and it is widely recognized among practising Christians that we are all flawed and prone to "sin". When does a self-described Christian cease to become a "true" Christian through tendencies they widely recognize as part of our human nature? According to Biblical definition, lying is immoral, yet it is quite difficult to find a single person who has never told a lie. Even assuming this to be a "forgivable" immoral act, one can invoke myriad other acts, from infidelity to theft, that we know are perpetrated by many self-described Christians.
The claim, “Such-and-such immoral people are not true Christians,” seems to be based a poorly formed notion of what is necessary to be a Christian. Without definitional clarity, it is easy for an apologist to commit the type of faulty reasoning commonly referred to as the "no true Scotsman fallacy". This is when a person, as part of a group, attempts to differentiate himself or herself from actions of another group member by arbitrarily defining the group such that it excludes that member. An example would be the assertion by a Scotsman, upon seeing another Scot playing a saxophone, “No true Scotsman would choose to play a saxophone!” The error here is that the traits actually defining one as a Scotsman include being born on Scottish soil, or being born into the Scottish culture, or being of Scottish ancestry; the type of instrument one plays has no bearing on this status. Similarly, a Christian is a Christian because he or she believes that Jesus of Nazareth is the Son of God who came to Earth and died to atone for the sins of humanity. Whether one is a good person or a bad person is independent of this belief, as behavior is clearly not the characteristic that defines Christians as Christians.
Intellectual honesty requires atheists to acknowledge that there are some immoral atheists. Likewise, intellectual honesty requires Christians to acknowledge that there are some immoral Christians. Ultimately, this common retort to atheism only distracts from the question under consideration, which is whether there is reason to believe positive claims for the existence of a God.