Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. (Matthew 7:21)
You are a Christian if you try to follow what you believe Jesus asked of people - if you try to live according to your understanding of his words and the will of the Father. It isn't about what you say - or even about many of the specific, theological details of what you believe. It's about what you do and what that says about what you really believe, in practical terms. It's not about being a theologian or a "believer". It's about being a disciple and a "follower".
To me, it's that simple.
As an aside, in our 5th Sunday lesson a couple of years ago, one of the men in the ward mentioned how much easier it is to be a good Mormon than it is to be a good Christian. He said it calmly and introspectively about himself, and nobody batted an eye. I really like the thought behind his statement - that it's harder to live the life Jesus asked of his disciples than it is to obey a checklist of do's and don'ts that gives an outward appearance of discipleship.