The answer is not showing that some passage in, say, the Old Testament isn’t accurate. (Many Christians don’t believe the writings that make up the Holy Bible are infallible in a straightforward way – instead, they require scriptural and historical context to understand what is being said, the proper emphasis, what purpose(s) the text might have, and so on.) Nor is the answer showing that some theological or philosophical conception of God isn’t quite right. (Christianity is substantially defined not by abstract theological or philosophical speculation, but by everyday experience.)
To answer this question, you have to ask what is at the core of Christianity. To make the question more complex, Christianity is significantly heterogeneous – different practices, different doctrines, different emphases, and so on, within different traditions.