I don't. There is no way of knowing whether he ever existed or not. All evidence is anecdotal and hearsay. It's probable that he did exist, because of how famous he is and everything, but there is no way of verifying this. He's also not real in any physical sense anymore, he's been dead for 2000 years. He is real in the sense that he has touched the lives of many and that there have been innumerable wars and atrocities committed in his name, but he's not real the way the moon is real, or the Sun is real. So if you know he's real, then you're wrong. You may be totally 100% sure he's real, but that doesn't mean you 'know' it for sure.
