While I don't doubt that this could've happened, I can only find one source for this, and saying that someone was "fired" from the army seems a bit fishy.
I'd assume that they mean either an 'other than honorable', bad conduct, or dishonorable discharge, but those don't get handed out lightly, certainly not for an ordinary religious conversion. Either the story's bunk, or Brooks did something dubious during or after his conversion.
I don't care who or what a soldier prays to as long as he'll pull the trigger when the time comes. Unfortunately, given this specific guy's words, I'm not certain he would.
Hit And Run Poster Extraordinaire
--Too Dumb to Quit--