I answered a question recently for the "pro" side of this argument because one of the main themes of this novel centers around redemption, and that is the foundation of Christianity. Therefore, it's difficult but not impossible to argue against the centrality of Christianity in this novel. There are other themes in the novel, so perhaps you should focus your argument on the fact that THESE themes are more central - such as the social aspects and democratic themes.
With regard to arguing against the centrality of Christianity, if you do some research on Victor Hugo, you will find that he was hostile to organized religion because he felt the Catholic church was not doing enough to help the poor (hence the social theme). The novel, in fact, was "banned" by the church. Hugo's religious beliefs changed dramatically during his life and he became very disillusioned and antagonistic toward organized religion. You could make the argument that a writer who was so opposed to organized religion could not possibly have Christianity as a central theme but rather, the more central themes of the novel were social.
Also, when Javert kills himself, he has allowed Valjean to escape and he is conflicted because the right thing to do is against the law, and Javert is a policeman who is supposed to uphold the law. Christianity believes that people are called to obey the higher law and that if the laws of a country are immoral, then it is morally wrong to obey them (as in the case of Nazi Germany - the laws under Hitler SHOULD have been disobeyed). So Javert should not have been conflicted by this, if he was truly following in the Christian way. Also, if redemption is available to all in Christianity, why didn’t Javert seek redemption? If he had, he would not have killed himself. He would have felt forgiveness, just like Valjean. Further, suicide was condemned by the Catholic church during the time of the novel. People that committed suicide were not allowed to be buried inside Christian cemeteries. These are some points you could use in your argument. Hopefully, this will spur you on to find other points.
As I recall from prior questions, this assignment is to be a debate. So in a debate, when you are on an opposing side and you don't have a stronger case, you must discredit the views of the other side. You also must be prepared to anticipate how the other side will attack your arguments. The opposition may claim that even if the ideas that I have suggested above are counter to Christianity, Christianity is STILL central in the novel, pro or con. Also, organized religion has been known to pervert the teachings of Christianity (the Crusades), so be prepared for these arguments.