I think it is more likely that John (Elder of Nazareth) was father to Thomas K. rather than brother.If John was a founding elder of the church in 1772 (when the church was officially organized), then his age would make him more likely to be father.
William N. McElrath relates in his book that family tradition that was passed down from as early as John's great-grandson indicates John was from Ayrshire, Scotland and migrated to Canada as a British soldier in the French and Indian War.Afterward he moved to PA then to VA and then SC.If I remember correctly (I haven't researched it) "Old John" was already in America by that time.So that tends to rule out "Old John" as father to Nazareth John.
I imagine "Old John" and Nazareth John were related somehow as well as to other NC and SC McElraths.There was a Michael McElrath from NC that Nazareth John was guardian of for a while and who moved to TN with him.I suspect they were fairly close kin.And there was another John who migrated from Northern Ireland in the late 1780's and settled next to Nazareth John (they are listed next to each other in the 1790 census).And in the early 1790's another Michael McElrath migrated from Ireland and settled near them also.I suspect the later John and Michael to be close kin, and suspect them to be probably nephews of Nazareth John.But I have no documentary evidence of relationship.It might be interesting to have a DNA project on descendents from each of these lines to see how far back to a common ancestor.
Nazareth John moved away from Spartanburg SC to Montgomery County, TN in 1798.Apparently, Hannah McElrath moved there with him and was married in Christian County, KY in Nov 1798.The marriage document explicitly shows her to be the daughter of John. This is the only documentation I have found proving a relationship to John.In addition, I suspect him to be the father of William (who later moved to TN and who bought land from him in 1812), Nathaniel (who stayed in SC having bought some of John's land), and Thomas K. (who went with John to TN or followed very soon after).