If I were a Christian who didn't believe in God.....
(*looks in the mirror*)
I have been this person. urluckyday is strictly speaking correct, you could not call yourself a Christian as it is commonly defined, a believer in the Christ as the Son of God.
The early Christians were not necessarily looking at Christ in this way... certainly not all of them, as there is a wide variety and interpretation of the lifetime and events of Jesus of Nazareth. Christ, Jesus, could have just been a cool dude that had great ideas. That is how the Buddha is seen.... and how some people choose to look at Jesus. There are many Buddhists who will also claim to be Christian (to an extent), because they believe that Jesus lived and made a great example - like the Buddha. Of course, many Buddhists practice religion quite differently than western minds are accustomed to - it is called syncretism, IIRC.
So, there could be a case of an agnostic individual who nevertheless likes good moral examples of living, and chose to model her life following Jesus' example and loving and following the life of Jesus, and still never for a day believe that Heaven is the reward.
Actually, come to think of it, that is my kind of Christian!