From an administrative point of view, the difference is subtle but important -
1. Proselytising
Someone whose motivation is to insist upon the absolute correctness of their own vision, which much be adhered to, promoted, and advanced, usually at the denigration of other viewpoints.
2. Witnessing
Can happen in various degrees, from the simple personal affirmation, to full-blown dictation of faith.
The problem arises where one form is exclusive of the interests of the audience, whilst the other is inclusive.
For example, Bobby Winters has a writing column on this site:
An Okie in Exile. He is a Christian, whose writings usually center upon a personal Christian approach towards secular issues. This means that he could be construed to be witnessing in some form - heck, some of his pieces are lay sermons, so you could even argue that he's preaching.
However, it is not an approach which derides anybody, attacks opinions or viewpoints that are different, nor dictates that others should take his outlook at their own. He makes a comment on the world, and invites not debate, but discussion and reflection.
This is precisely why I invited him to post writings here - I find them enjoyable, engaging, and often worth reading.
Big Buddy is a particular favourite, though there are others.
However, if he simply wrote that he had faith, this was his faith, and that others better take that same faith upon themselves, then he would not be enjoyable or engaging to read, but invasive.
Hopefully that helps with my own personal take on the matter.
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)