One common challenge to the idea of Christianity in general is the idea that many people are only Christian because they were born in a predominantly Christian society. This argument takes many forms. One common claim is that if the same person (who claims to be Christian) were born in Iraq, they would be a Muslim.
Other times, the claim might be more along the lines of the idea that someone is a Christian because their parents raised them that way. In other cases, they may attribute someone's faith to a reaction of some tragedy or trauma in their life.
Whatever the case may be, these and other similar challenges are examples of the "Genetic Fallacy." The reason this is considered a fallacy is because it makes the mistake of thinking that by identifying the origin of a belief renders the object of that belief untrue.
When one takes just a few moments of thinking about this, it becomes obviously fallacious. It is not even difficult to imagine scenarios in which the absurdity of this type of thinking can be made clear. For example, imagine you were with some friends and were discussing the most recent Super Bowl. At some point in the conversation, one of your friends mentions something about the winner of the Super Bowl. Immediately, another friend chides them by pointing out that they did not actually watch the Super Bowl and the only reason the other person believes that the stated team won was because he read it something about it on his Twitter feed.
The implication seems to be that since this person only has second-hand information, someone else tweeting the name of the winning team, that he cannot have any confidence in his belief about who won the Super Bowl. Well, this is obviously faulty thinking. While we might be able to perhaps claim that he does not have good justification for his belief, the source of his belief has absolutely no bearing on the truth value of that belief.
In a less fictitious example, I have known people who have worked on algebra problems in ways that I have no clue how they arrived at their final answer. However, while the method by which they arrived at the answer is suspect, when verified it turned out that the answer they arrived at was correct. Even if they did not use legitimate laws of mathematics, that did not change whether or not the answer they got was correct or not.
The fact is, you simply cannot link the truth value of any belief or claim with how that belief came about. You can certainly challenge whether or not someone is justified in holding to a given belief, but how they came to that belief has not connection to the truth of the belief.
Hopefully, that is clear.
For more information on the Genetic Fallacy: