Proponent comes from Latin proponere "put forth, set forth, lay out, display, expose to view," figuratively "set before the mind; resolve; intend, design," from pro "before" + ponere "to put".  

According to the Oxford English Dictionary (OED), the word "proponent" was first used in English in 1588 in the writings of Abraham Fraunce, a poet and lawyer. 

Example of "proponent" in a sentence: 
"She is the most high-profile proponent of a theory that video games have the power to heal." 
"Is becoming an Eagle Scout a waste of time and energy?  Not at all, say its proponents." 
"He was a leading proponent of a rational, sensible approach." 
"There, he was an early proponent of the armed attack helicopter." 
"The mayor is a strong proponent of recycling."