On the other hand, I've met many, many Christians whose Theological depth barely reaches past about a dozen Scripture verses that they can quote (or misquote) by heart. They may be able to name the books of the Bible, but they certainly haven't read most of them. I'd be interested to know what percentage of the Christian world has read even one single book of the Bible straight through... even 3 John, which is only 15 verses long. I don't have any real statistics, but I wouldn't be surprised to find out that even 50 or 60 percent of Christianity has barely even opened a Bible. One of these poorly-read Christians is no match for an opponent who has spent any amount of time studying Christianity.
Among Christians who do actually know something about the Bible and their own belief system, an astounding number disagree on any number of things, even things they consider quite fundamental. You would think that adherents to a faith based on the contents of one single book would be much more unified, but apparently there is a lot of room for interpretation and reading between the lines. So oddly enough, two Christians debating some point of Theology with an atheist might even find more common ground with the atheist than each other.
There is only one thing that can convince someone who is antagonistic to religion of the existence of Christ: a Christian who is truly living out his faith. Two Christians that disagree on speaking in tongues, or faith healing, or transubstantiation, or predestination, or any number of other topics, will react to life situations in almost exactly the same ways when they shut up talking about doctrine and start living like they believe Christ would have them live. Honest-to-goodness Christians who are living their faith and not just talking about it are going to think alike and act alike. We know in our hearts how Jesus meant for us to live. And a life lived for Christ can't be refuted with clever intellectual arguments. A life truly lived for Christ is undeniable. Two Christians who disagree on doctrine but whose lives match because they live for Jesus constitute pretty convincing proof that something unites them. That something is Jesus.
"...By this all people will know that you are my disciples, if you have love for one another.” (John 13:35 ESV)