A common fallacy of argumentation that you’re bound to encounter is that of circular reasoning.
Circular reasoning (similar to begging the question), know by it’s Latin name, Petito principii, is a fallacy of argumentation in which the one arguing states as a premise, that which he is seeking to establish as a conclusion.
Example: “Professor, you can’t give me a C, I’m an A student!” In this example the student assumes (I am an A student) what she wants to prove (You can’t give me anything other than a A).
Christians are sometimes guilty of this fallacy, arguing:
Premise 1: The Bible is the word of God
Premise 2: The word of God cannot lie
Premise 3: The word of God says that God exists
Conclusion: Therefore God exists
Asserting that the Bible is the word of God can only possibly be true if God exists. If God did not exist then whatever the Bible is, it would not be the word of God.
Atheists can also be guilty of this fallacy. For example:
Premise 1: Nature is all there is
Premise 2: If God exists he exists beyond nature
Conclusion: Therefore God does not exist
Here’s a short excerpt from a debate between William Lane Craig and John Shook in which Shook is caught in circular reasoning:
What are some examples of circular reasoning that you’ve encountered?