" Letter was only to Corinthians not all Christians."
Exactly !!! when Paul wrote to the Corinthians, only a few groups of his followers existed, the so-called "Governing Body", which it wasn't, no such thing existed in the 1st Century, but the Elders in the Jerusalem church were probably unaware of these Epistles for years.
The Org. also use verses from the Letters of John, for the D.F policy, but take these out of Context, those verses are talking of Christians who deny that the pre-existent Jesus came in the flesh, the Writer calls them for this reason "the Antichrist".