How did Adage I'm beside myself originate?

Similar to "I'm out of my mind". Originally came from a phrase used in the Bible, in 2 Corinthians 5:13 - "For if we are beside ourselves, it is for God; if we are of sound mind, it is for you." This refers to how a truly dedicated spiritual person interacts directly with God (with great, seemingly overwhelming passion) vs. how they react to people (in a more measured, focused way). It did not originally have a bad connotation, but it has evolved (as many words and phrases do), over the years, to mean someone who is unstable or "not right."