Of all the logical fallacies in the world, this one is perhaps my favorite. More than any other fallacy, this seems to be the one people commit when critiquing the Christian world view. In fact, I cannot think of a single debate, discussion, blog post, article, interview or other interaction in which someone has accurately represented the view of any Christian I have heard of.
And that is the Strawman Fallacy. Misrepresenting the opposing view in order to easily defeat the view which said opponent does not actually have. It is often quite subtle. Mainly because to use an effective strawman, you need to make this "fake" view look very similar to the real view so that people will think you're defeating the real thing.
Let's say, for example, you are in a conversation with someone and they ask you how you think you're going to get into heaven. During the conversation, you end up bringing up John 14:6, the famous "no one comes to the father except through me" passage. Your opponent shoots back at you, "So, you mean to tell me that this all-loving, all-good, all-powerful God of yours would send me to hell forever just for not believing in Jesus? That's awful! How could you believe in a God like that?"
See what they did? They basically tried to say what Christianity says about salvation, but they twisted it a bit so that it doesn't actually represent your view. If you're not on your guard, you just stand there, flat-footed without a response. If you catch on to what's happening, you could respond by challenging and pointing out the straw man.
Here's what you might say...
"Actually, I must not have made my view clear. Let me clarify. The reason people are condemned to Hell is because of sin. As it says in Romans 3:23, 'for all have sinned and fall short of the glory of God.' That's the reason people go to Hell. Not because they don't believe in Jesus, but because we're fallen creatures in rebellion against an infinite God. So, it's not the lack of belief in Jesus that condemns us, it's the belief in Jesus that saves us.
To reason that because belief saves us that therefore non-belief condemns us is an example of a Straw Man. It would be like saying that since taking Tylenol reduces fever, not taking Tylenol causes fever. Obviously, we know that this is not true, but it's the same logic used in the example of the straw man above.
Whenever discussing opposing beliefs, it is important to accurately understand and represent your opponents view. Otherwise, the only thing you will defeat is a straw man, not their view. In the end, you've won nothing and lost credibility.