The Emerson quote was one of my father's favorites who like Emerson, was a Unitarian minister. My father used it in at least one sermon where he taught that religion should always be independently validated by each individual and we should never take for granted what someone else tells us to believe, or not believe. When asked what to believe he would always say it's up to you to figure that one out. I'm not sure if many religionists who are not Unitarians or subscribe to certain far eastern theologies would care for that philosophy much, though.
As far as all those people you mentioned, with the exception of one we have independent verification the writings(or at least the vast majority of writings) attributed to them are genuine. So it makes it a much simpler matter to derive understandings of their works. In the case of Jesus, some of the biggest debates among theologians concern whether biblical passages can actually be attributed to him, and biblical historians often debate whether any of it can be.
Anyone can be misunderstood, so I don't really attribute that quality to greatness. YMMV.
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