There is a fundamental difference between believing in God and experiencing the presence of the Heavenly Father within oneself. "The kingdom of Heaven is within you."
The only justification a theist can offer is his personal experiences from his belief. Or the experiences with his belief justifies his belief. This is rational to him. For a non-believer, the non-believing experiences justifies the non-believing. The believer looks the reality from inside out...
You can not reason God into existence. God is existential, without beginning or ending. To God all is an infinite now. In addition, there is some part of existence may be without beginning and ending. Or co-eternal with God.
Personal religion: each individual discovers God by himself as himself and for himself. All religionists went through the same process, they comprehend each other and love each other. There is no more organized institution of religion such as Christianity.
Jesus is a Son of God coming from above to demonstrate how man can become divine (perfect). He is God and man by the virtue that he is from above, a Son of God incarnated in a human body.