What is the argument against the authenticity of 1 John 5:7-8 in the KJV?

I was reading through Lee Strobel’s The Case for the Real Jesus and came across a passage in the book that indicates the inauthenticity of 1 John 5:7-8 in the King James Version: 1 John 5:7-8 (KJV) 7  For there are three that be…