It's impossible to overestimate the influence of Napoleon Brown Culp. It was he that brought hard core gospel-style vocals into the secular realm, for all practical purposes inventing "deep soul".
I don't really need to write anything else 'cause that about says it all, but there's something of interest I'd like to point out. Much has been written about Brown's voice, his use of vibrato/melisma,/etc.....but I've yet to see anyone mention that the man regularly sang in what you might call a mid-range falsetto. His natural voice being very deep, sounding something like John Lee Hooker with considerably more lung power.
You can hear the shift between his natural voice and the falsetto on almost every one of his records. Quite suddenly he'll drop an octave [or two] time and again. At it's most exaggerated, this vocal shift can make the records almost sound like the work of a non-harmonizing duo.
On this cut, Brown starts off singing in the falsetto, then quickly reverts to his natural voice [pushing it about as far as it'll go], and then shifts back to the falsetto for a few seconds before finishing up the record in his natural voice. The shift isn't as obvious as it is on some records, but it's definitely there.
The word for today is, fascinating.