You accosted dude on some "you should be witnessing to me bro", but then you tell him you're an ex-christian and atheist who will never believe again, which means his instinct/intuition/or whatever was correct to just walk away from you. He could've been witnessing to others but just not to you, in particular, because the "spirit led him not to" as some christians might say.

You seem to want to present yourself as a stand-up guy, and that's cool, so I'll say that it would have been more honest of you to tell him to hold on because you wanted to share some secular, every day wisdom with him... that you don't think all that christian and bible stuff is necessary. That's essentially what your purpose was, but you didn't own it and chose to pass it off on him by calling him a dishonest rep for the ministry.