There is no certain way of being logically a Christian, even if God's existence is practically self-evident. Christians are effectively just a brand of people who believe in God; and they have added on to God this 'Son', Jesus Christ. They say that their Jesus is God, or the Son of God. To get to the point, a Christian holds that Christ was God's Son, also consubstantial with Him. And, as proof, and inherent to his nature, he arose from the dead.
That is to say, if he did not rise from the dead, and was not seen by those same people (historically speaking, an obscure group of individuals), if he was not witnessed to have survived death, then there certainly would still be God for Christians, but there would be no Christianity. It is the death, then the return from the dead, which characterises the very heart of this religion, distinguishing it from every other religion or cult. Christ, so to speak, in this act above all, defied nature, and sin, and so on. Everything else follows.