Modus ponens means "way that affirms by affirming" and it has a formal statement in terms of P and Q (which you can find on the wikipedia page), but it basically says that if some proposition definitely entails another proposition, but that second proposition is false, then the original one is false also. For example:
If I always have a cold during winter, and I do not have a cold, it follows that it is not winter, by modus tollens.
How does this apply to Christianity? A few days ago I wrote a post entitled "Why the Old Testament Cannot be Waved Away", and although I tried to simultaneously point out why it cannot be ignored and why it cannot be taken at face value...some things just seem obvious. I can imagine that with some form of Old Testament authority in mind, one can construct at least three types of modus ponens or modus tollens arguments:
Now, all three will need to be examined individually by each Christian (when actually formulated, of course). Anyone who is going to consider the Bible as a source of information about God will have to wonder about the arguments against it at some point if they are to be rational beings. I shan't formulate them now because I want you to think for yourself whether there is anything in the Bible that could disprove its validity. I can think of at least a few contenders.