Begging the question is the most common logical fallacy. Begging the question is also known as Circular reasoning.

One example is this, "Evolution is true because it is a fact." The Premise (part a) merely restates the conclusion (part b).

Now you gotta be very careful when you pull this up, the atheists have a very tricky answer to this. This also connects to the Morality argument. The atheist will say something like, "You believe the Bible is true because the Bible says so! Hypocrite!"

This is true. Christians do, unfortunately, commit the fallacy of begging the question. But, there are two things you can do about it.

#1: We have an absolute standard (See the morality argument) With and absolute standard, the premise and the conclusion must agree.

#2: You should try to convince the atheist first that God must exist in order for us to exist. Then when we use our ultimate standard, We aren't begging the question.

I hope it's not to difficult to understand.