I have a bumper sticker that makes fun of a common religious bumper sticker. It looks very much like the more famous one, but it says "Religion stops a thinking mind!" Of course, this is only used for shock value. I'm well aware that people who are religious aren't mindless zombies - under most circumstances. However, there are times when their eyes glaze over and the ability to think is tossed overboard faster than they tossed Jonah into the ocean.
In a conversation, most of the time, you can discuss any subject with a religious person, and they will use normal thinking skills, using their ability to rationally approach the subject at hand. They have their opinions on any subject other than religion, and if they learn something that conflicts with what they thought was true, they accept that they were wrong and internalize that fact. I have often been in conversations with deeply religious people who had simple misconceptions about the subject of astronomy, for instance. Those who don't make a hobby of learning about the universe have lots of bad information about it - like the idea (supported by popular songs) that the north star (Polaris) is the brightest star in the sky - which it emphatically is not. Or that there's a "Dark Side" of the Moon, a place of perpetual darkness, which also is not the case. All I have to do is point out the location of Polaris, and several brighter stars to get the first point across. A couple of minutes with a ball and flashlight to demonstrate the other. Once the point is made, there's no need to worry about the truth of the matter.
This process of rational analysis will even continue when they discuss any religion that isn't their own! I know many thoughtful people who have no problem with finding the logical errors with other religions. Some take great pains to find every shred of evidence possible to prove how wrong other religions are. I once was talking with a person who is a very thoughtful Christian. I mentioned to him that I thought the Muslim religion was just as valid as his. Of course, he took exception to this. The interesting thing about his reaction was that he then went into a list of reasons (nearly all of them valid) as to why Islam was not worth believing, including problems with the Koran, the less savory items in the life of Mohammed, and reasons why there could be no valid relationship between Islam and Judaism, as the Muslim claim goes. It was all very admirable.
However, any time I talk with this same person about any of the various logical problems with Christian beliefs, and the sparks start to fly. I once mentioned that the first two chapters of Genesis contradict each other on several points, especially concerning the issue of the order in which things "popped" into existence. I was told that two different people will see an auto accident from different perspectives, and that those will agree on major points - in this case, the two accounts agree that God created the world and all living creatures, including humans. This is an ad hoc argument, and a sloppy one at best. At other times, I have asked the same person about who wrote the book of Genesis (as in "who actually put the words on paper?"). Many believers, especialy fundamentalist Christians and definitely including my friend, believe it was written by Moses (they point out that the first 5 books of the Bible are called "Books of Moses" and that Jesus said they were written by Moses), which sort of shoots down the theory of "two observers of the same event" from the discussion I mentioned earlier.
Now, the question becomes, "Why should this shutdown in thinking occur?" In my opinion, it all boils down to the concept of "faith." Faith is presented to all religious people as a virtue, a thing most pleasing to God and impossible to live without. It also just happens to be the one thing that religions can't exist without. For faith is a mindset, an ability to believe in something in the absence of evidence that could support that belief. In many instances, faith also entails the ability to cling to a belief in the presence of evidence that the belief is wrong. Thus, any questions that can be raised, any contrary evidence is all ignored, in favor of following one's faith. In other words, religions order followers to shut off the rational parts of the mind and accept everything about the religion without using any rational mental processing. This is particularly true in the Christian Bible:
1 Cor 3:18-20 - Do not deceive yourselves. If you think that you are wise in this age, you should become fools so that you may become wise. For the wisdom of this world is foolishness with God. For it is written, "He catches the wise in their craftiness," and again, "The Lord knows the thoughts of the wise, that they are futile."
This leads to the comment about religion stopping a thinking mind.
In my opinion, this is a harmful situation. I consider the ability to take a proposition on faith to be at least an abdication of one's duty to oneself. If someone tells you something, presenting it to you as a fact, it is always prudent, if you're not familiar with it, to do some investigating. Going a little further into the faithful mindset, I have seen it become a regular habit with some people. Superstitions, urban legends, all sorts of rumors are accepted as truth, often without questions of any sort. There's nothing harmful about that much of the time, but that's not always the case. The real harm, however, comes when the con artists arrive. Any con artist practices his art by concocting a plausible story that makes the victim want to participate financially. With a habit of believing in things without looking into the evidence, people of a religious mindset can be easily taken in. This isn't to say that religious people are easily fooled because they believe in God - only that with an established habit of believing things simply because they want the things they believe to be true, that habit can lead them to make extremely poor decisions with disasterous consequences.
We then come to another problem with the religious mindset - the faith that God loves and protects a person can (and often does) lead a person to think he or she can live life using God as a sort of metaphysical net over which to perform all sorts of dangerous stunts. I have known many people (and I was one of them) who personified the statement, "Give a man a fish, and he eats for a day. Give a man a religion, and he'll starve to death while praying for a fish." My personal experience with the Christian religion leads me to the opinion that it, in particular, is dangerous in this regard, as reading the Bible will lead to finding the many passages where followers are promised all sorts of divine gifts and supernatural protection from worldly disasters. As a matter of fact, I would be far from surprised to find out that one of the more common problems faced by Christian ministers is dealing with this very issue. Luckily, they have an arsenal of Bible quotes that contradict those verses, which I like to call the "pie from the sky" passages.
In all, though, it's easy to see that religions and the faithful mindset that they all appear to encourage are harmful to the followers in many aspects of everyday life. The times that faith may be helpful are few and far between, and under nearly all circumstances it is useful only to the religious leaders who are well-trained in its uses. Is thinking and rational inquiry harmful? Not for any individual. It is seen as harmful and detrimental by religious leaders who work at their hardest to stamp it out when it appears in their flocks of intelligent sheep. If you don't believe me, just try asking some of James Buckner's questions to a church minister - and watch the sparks fly!