If anyone asks why I believe in Jesus, I am going to have to give an unacceptably abstract answer which may even appear irrational.
That is why when I was a young Christian, I sought to defend my faith by researching into apologetics, looking for historical and scientific proof that support the Bible's teachings about God. "Evidence that Demand a Verdict" by Josh McDowell, for example, painstakingly tracks archaeological evidence, historical records and personal accounts documenting the events in the Bible. To some extent, they helped me to recognise that the reliability of the Bible. Yet the truth is, it is very difficult for anyone to carry out debate of robust quality on this topic, since most of us do not have the requisite expert knowledge or access to primary data that should form the basis of our arguments. Neither are we willing to spend enough time reading adequately to have an informed stand, and hence all these debates tend to be severely guilty of straw man fallacies.
As a child, I remember looking into the sky and thinking that there must be a divine being. I prayed to him that the school dentist would spare me, and reproached him whenever the dreaded check up did happen. Yet who this God was, I did not know. Years later, when somebody introduced me to Jesus, my heart quickened with joy. Unhesitatingly, I knew Jesus was the unknown God I had been praying to, just like John the Baptist who leapt in his mother's womb upon meeting Mary (Luke 1: 41). It is impossible to explain the tugging in my spirit in rational, scientific terms, but is it any less real because our bodily senses are not involved?
Why should the physical be elevated over the spiritual as the source of truth? Bear in mind that the insistence on physical proof itself is a relatively new paradigm that has come about because of the scientific revolution in the past few centuries. In Platonic philosophy, the material world is corruptible and transitory, subject to change. Only "forms", or abstract ideas which capture the essence of reality, are permanent. Though Plato pre-dates Christ, I've always thought it is such brilliant representation of our earthly body, and the new, incorruptible one in the world that is to come. Yet in the modern scientific mind, we choose to disregard anything that cannot be empirically studied, and what loss it is.
If not through our human senses, how is anyone to come to know a God who is a Spirit being? I suppose He appears to anyone who is looking for him. The Bible promises that he who seeks finds, and the one who knocks will find the door open to him (Matthew 7:7). No one who truly wishes to know God would be denied, and according the same principle, Jesus spoke in parables so that only those who choose to listen will understand.