The concept of practical atheism has been 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 atheism' is thus intended a smear against atheism generally.
In Atheism: The Case Against God, George H. 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 Maritain’s 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 one’s 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?
The concept of "practical atheism" is also used as a smear against secularism when religious apologists argue that there is something fundamentally immoral and godless when religion is not permitted to control civil institutions.
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.