Have you ever heard of "practical atheists"? This is a category used by some religious theists to describe all those theists who technically believe in a god, but who behave immorally. The assumption is that moral behavior follows automatically from genuine theism, thus immoral behavior is a consequence of not genuinely believing. Theists who behave immorally must really be atheists, regardless of what they believe. The term 'practical atheist' is thus a smear against atheists generally.
In Atheism: The Case Against God, George Smith writes:
Practical atheists [according to Jacques Maritain] believe that they believe in God (and... perhaps believe in Him in their brains but... in reality deny His existence by each one of their deeds.
To state that men believe in their brains is a confusing way to acknowledge that they do, in fact, accept the existence of a supernatural being. By any rational conception of theism, such persons are theists, pure and simple. They may be hypocritical theists, they may profess to be Christians while ignoring Christian morality, but if these men actually believe in god in their brains (meaning: as an intellectual issue), then they are theists, regardless of their conduct or moral beliefs.
But the idea of a hypocritical Christian offends Maritains sensibilities. The belief in god is morally good, and the theist who does not measure up to certain moral standards then somehow does not really believe in god. As to how one can become an atheist through ones actions, Maritain provides a simple answer: if one is sufficiently immoral or hypocritical, one deserves to be called an atheist. Under the cloak of classifying, Maritain purifies theism by pushing its undesirables into the atheistic camp, where he has no difficulty accepting their deviant behavior.
After all, what more can one expect from a godless man?
Indeed, what can you expect? The concept of "practical atheism" is also used as a smear against secularism when religious theists, or perhaps we should say theocrats, argue that there is something fundamentally immoral and godless when religion is not permitted to control civil institutions. Both the smear against atheists and the smear against atheism stem from a surprising source: neutral dictionary definitions.
A few dictionaries, spread out from the late 19th through the late 20th centuries, include in their definitions of atheism a listed for "practical atheism" which is defined as "disregard of God, godlessness in life or conduct." This neutral explanation of practical atheism corresponds to current usage of the term godless, a label which covers all atheists and a few theists who don't bring considerations of what a god might want or have planned when making decisions in their lives.
There's nothing inherently negative or immoral about this, but it may be treated as negative and immoral from the perspective of religious theists who think that their god's desires and plans should be incorporated into everyone's decisions all the time whether they like it or not. A theist who behaves immorally, at least according to some observers, is a theist who is "godless" because they are not doing what (it is assumed) God wants and so they are "practical atheists." A secular system is one which doesn't consult religious leaders on what God wants and is thus a form of "practical atheism."
At one time a primary definition of "atheism" in most dictionaries was "wickedness." Atheists were defined as being inherently wicked, immoral, untrustworthy, and just plain bad people. Today most dictionaries don't even list this and if they do, they mark it as obsolete. Although usage of this particular word may be obsolete, the attitude behind defining atheists as immoral hasn't gone anywhere. People who want to smear atheists as immoral simply hide behind the phrase "practical atheism."