The NKJV footnotes erroneously point to the 'Majority' text, when they are in fact only referring to the faulty Greek Text According to the Majority (1982) by Hodge and Farstad. It falls far short of a full collation of manuscripts since it is based primarily on Von Soden's collation of only 414 of the over 5000 manuscripts.
There are a few verses which the unorthodox Greek Orthodox church did not keep in their Greek manuscripts. Therefore their history must be traced through other language groups. Dr. Moorman shows that those verses are strongly attested to by numerous other ancient language Bibles and authorities (e.g. 1John 5:7).