If Jesus is real, why do Christian's say they believe in him. Shouldn't they say I know him?
How can one believe something they know?
Firstly, everything about our faith is ultimately about the heart, our heart and its responses to whatever we face in life, good and bad, and, of course, to the communications we receive from God, the angels, saints and Holy Souls (the latter, at least, as sometimes people close to their final death attest to, as well as near-death experiencers. Hence, mere credence is of scant significance, and need not be a gauge of the generosity of our heart. As St James remarked : 'The devil believes and trembles.'
Actually, most atheists are devout believers in the sense of mere credence, whatever their protestations, and vociferously express their deep displeasure with God because it appears to them that He doesn't seem to be compassionate, what with all the innocent suffering in the world - which is probably the most difficult criticism that Christian apologists have to counter.
However, the deepest spiritual truths, those of our faith, reaching up as high as the Most Holy Trinity, God, himself are increasingly paradoxical, and actually repugnant to reason. We tend to call them 'mysteries', because they are so unfathomable by our worldly analytical intelligence, such access as we are allowed, being granted, instead, proximately at least, to what has been called the 'unitive intelligence', which all mainstream religious traditions have identified as the province of the heart. And that means 'of the will' and of wisdom; the philosophical school of metaphysical voluntarism.
Consequently, it can be said that we actually do believe in what we wish to believe - in the Christians' case, that the life and teachings of Christ were/are sublime and worthy of belief (because by God's grace, they make sense to us) and , moreover, following from it : commitment to Christ's mission to live according to his moral and spiritual values, and to evangelize all of mankind.
Another way of looking at it, is to consider secular faith as its analogue. When we enter a room of an evening, we switch the light on. We don't know for certain that the light will come on, but we believe that there is there is a very, very good chance that it will. In a similar way, Christians check things out and interpret them in the light of our faith, usually positively.
On the other hand, atheists and some agnostics, when faced with 'slam dunk' proof pointing to a Designer and Creator of the universe, and any and everything else that might exist, and to other truths we can take for certainties, simply will not, will simp;y ignore the point and move on to another topic. Why ? Because they fear, rightly(!), that they would have to change their world-view and life-style. They do NOT want to commit to a religion such as Christianity, which would make major demands on their way of life. So, they do not find in Christianity, the extraordinary beauty that we Christians do, though their culpability lies in the refusal of their heart to defer to what ought to transcend their selfish carnal world-view. most notably, Christianity.
Dictionaries define a continuum as a 'confusion of concepts', such as space and time. I believe that, without Einstein's great insight, philosophers would say that trying to fuse them would be a 'category' error. However, as a lens through which to view time and space as the continuum we now know it to be, faith and knowledge as a continuum seems a perfect fit.
Little wonder , then, that, while other great countries such as China and middle-eastern countries made occasional scientific advances long before we discovered the same, it was only Christianity that really set science in motion as an ongoing project, prompting its adherents to believe in a divine law-maker and hence a Creation that would answer to certain physical laws that were rationally conceived to be understood by our rational minds, because its Creator was the supreme source of all Reason.
Scant wonder, too, that not one atheist has been responsible for a major paradigm-shift in physics. Those who were, such as Galileo and Newton, were not just routine Sunday worshippers, but exceptionally devout men. Only the power of his father was able to prevent from Galileo becoming a priest - but it doesn't suit our corporate, atheist media to broadcast that.