Often found in the comments section of forums and especially YouTube. A Keyboard Preacher is an individual who espouses religious and god related material. The preaching is more often than not completely unrelated to the conversation at hand.
Keyboard Preachers often talk about nothing other than god and religion and feel compelled to share their religion with others regardless of whether their audience cares or not. Keyboard Preachers will often cite scriptures and verses from the bible.
Example of Keyboard Preacher
Comment: "Joe Biden is completely inept at his job"
Keyboard Preacher: "everyone will kneel before Jesus Christ and all tongues will confess to come unto the lord"
Commenter: Nobody cares. Quit being a Keyboard preacher