A Christian that does not believe in Jesus is frankly not a Christian. I say this with confidence because the foundation of Christian beliefs are rooted from the Bible; within the Bible it is stated MANY times in the New Testament that in order to commune with God you must go through Christ. So the question I would have for someone who calls themself a Christian but does not believe in Christ is "Do you believe the Bible?"
A scripture to illustrate is:
John 5:23 "that all may honor the Son just as they honor the Father. Whoever does not honor the Son does not honor the Father, who sent him." (NIV)
This is a perfect example of God's will for all to come through the Son. In order to get the full context of the scripture I've listed the preceding and proceeding scriptures below - verses 22-24.
22"For not even the Father judges anyone, but He has given all judgment to the Son, 23 so that all will honor the Son even as they honor the Father. He who does not honor the Son does not honor the Father who sent Him. 24"Truly, truly, I say to you, he who hears My word, and believes Him who sent Me, has eternal life, and does not come into judgment, but has passed out of death into life.…
I'm not sure if you wanted a philosophical response however I believe that is why so many people are confused today. It goes back to the question I posed at the top of this post.....do you believe the Bible??