I think everyone should see a marriage counselor before they get married even if everything is going fine because there are many potential sources of conflict that are good to discuss beforehand that they might not think about. For instance, one of both of people might have strong feelings about whether a potential child goes to a private or public school and it is better to work things like that out before you're approaching a deadline where you have to make a decision and find out that you're not on the same page. However, I would especially advise this when you're already not on the same page about something that is important to her.
Many Christians think that Christians shouldn't get married to non-beleivers, so she likely will get advice from other Christians not to get married to you, or might have friends and family who wouldn't be happy with her decision. The decision to get married is up to you and her, but the reality is that there are people you would have to interact with. Then again, it might be the case that they approve of you, but that is a potential problem that should be considered.
Another issue might be whether either of you have hopes of changing the other. Does she have hopes of you converting to Christianity? Will she become put off if she sees that reject Christianity? Have you investigated whether or not Christianity is true? Becoming a Christian would alleviate this tension, but you should not become a Christian in order to please her, but because the evidence most strongly supports that it is true.