We use Wikipedia's guidelines to determine if someone is considered "Notable". Their definition states:
On Wikipedia, notability is a test used by editors to decide whether a given topic warrants its own article. For people, the person who is the topic of a biographical article should be "worthy of notice"[1] or "note"[2] – that is, "remarkable"[2] or "significant, interesting, or unusual enough to deserve attention or to be recorded"[1] within Wikipedia as a written account of that person's life. "Notable" in the sense of being "famous" or "popular" – although not irrelevant – is secondary.
So in looking at William, he was an activist for his time, which is somewhat notable, he was a publisher of a controversial book, which is notable, and he was with a group who took a stand on slavery at a time when it wasn't necessarily the popular thing to do - also notable.
Generally when I'm on the fence with someone, I would tend to lean towards making them a notable rather than trying to restrict them. I've seen weaker cases for notability OK'd by Wikipedia, and can picture a nice Wikipedia article on him based on what I've read. Unless someone has a strong objection, I would consider him a Notable.
