How do you know he did? Because he claimed to? Because the "new testament" says so? Because chr*stianity believe this? Because it is Catholic dogma?
BTW, I hope you also have a "Damascus Road" experience and one day come to acknowledge the existence of your forefather Adam.
How can we know anything? I believe it because the Church teaches it, and because the reasoning behind the teaching has convinced me of its truth. If you want more than that, sorry.