--Special Agent William Hagmaier of the FBI Behavioral Analysis Unit quoting Ted Bundy (serial killer)
"Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment: But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment:"
--Jesus
Some people describe religious beliefs as "false yet harmless" convictions lacking any real or demonstrable harm to the person holding them to be true. But can believing something to be true without supporting evidence really be without harmful effects on other aspects of one's life?
While some thoughts or beliefs, at least on the surface, may seem not to directly hurt a person, the very process through which one comes to believe falsehoods is harmful in itself, and if left unchecked, can sprout through in other lines of reasoning until one does, in fact, believe something harmful.
For example, let's say I believe Santa Claus exists and I have some anecdotal, though fairly unconvincing evidence to support this hypothesis. While the specific belief that "Santa is real" may not hurt me, the fact that I accepted the claim without thorough examination, perhaps because it makes me feel comfortable or offers me reassurance in some way, leaves me susceptible to accepting other unsubstantiated claims on tenuous evidence. It is the process of thought which causes the harm more than any specific false or unsupported belief.
For example, let's say I believe Santa Claus exists and I have some anecdotal, though fairly unconvincing evidence to support this hypothesis. While the specific belief that "Santa is real" may not hurt me, the fact that I accepted the claim without thorough examination, perhaps because it makes me feel comfortable or offers me reassurance in some way, leaves me susceptible to accepting other unsubstantiated claims on tenuous evidence. It is the process of thought which causes the harm more than any specific false or unsupported belief.
Throughout the countless religion classes I attended while growing up in the Mormon Church, I heard several times that "our thoughts inform our actions," or some variation of that theme. For this reason, one should be mindful of one's thoughts since thinking about bad or sinful things (especially things of an explicit or sexual nature) will invariably lead one to act accordingly. Cases of serial killers and rapists are often described as examples of people who contemplated about such horrific acts long before committing any crimes, thus suggesting that in some sense their crimes could have been avoided by thinking happy thoughts.
But it was not the thoughts of Ted Bundy, and others like him, which led him to kill so many people. No; his beliefs, not simply his thoughts, informed his despicable actions. If he had merely thought about killing people, rather than believing that he could and should do so--consequences being damned somewhat ironically--he likely would have lived a long life as a charismatic, if slightly off-centered and perverted loner. Additionally, being a sociopath, thus lacking the necessary empathy to prohibit him from inflicting harm onto another human being, didn't help.
Likewise, if I spend a great deal of my time fantasizing about flying like a bird or Peter Pan, I know deep down that any attempts to do so would be foolhardy and no amount of thinking will allow me to actually attempt it successfully. However, if I sincerely believe I can fly, perhaps only under specific circumstances (i.e. with the aid of pixie dust), the game changes entirely, and I might be inclined to try it.
Many suggest reciting or humming a religious hymn as an effective way to distract oneself from unseemly or distasteful thoughts and thus save oneself from the anguish derived from the eternal effects of unrepentant sinning. Coincidentally, many also discover the very Pavlovian conditioned response which may occur if one repeatedly couples thoughts of hymns and sexual desires, however innocently or well-intended.
As compelling as the argument may sound that our actions are so easily informed by even our worst, most depraved and perverse thoughts, fundamentally the reasoning is flawed, and is nothing more than a feeble attempt by small-minded theocrats to control the minds of their adherents--especially youth with budding libidos. This is what George Orwell described in his book 1984 as "thought crime". It is not enough to say that one should abstain from sexual promiscuity--even thinking about something like adultery, according to Jesus, is the same as committing it, and is just as damning. So much for nuanced and thoughtful consideration from a "just and merciful" god.
Bonus Material:
The Atheist Experience dispelling the notion that lust is the same as adultery: