So goes the catechism. How do they know that?
I thought the article did a good job of outlining why it was fairly possible of his Pharasiacal roots. It is long, but if you get a chance to read through the entire article I think you will find it interesting.
Circumstantial evidence indicates he might not have been Jewish.
Which circumstancial evidence would that be? I think the article outlines in detail exactly where his teachings came from.