When John wrote this, the Church was being infiltrated & overrun with false teachers that were denying the very fundamentals of Christianity such as Christ’s deity and His resurrection. (Much like we’re seeing now with the Progressive Movement of Churches) John wanted to make it clear…he had personally saw, heard, and touched the Son of God…not just some good man he had hung around with for three years. He was an eye witness to miracle after miracle…he had heard Jesus rebuke & demand evil spirits to leave tortured bodies…and he had enjoyed close fellowship & comradery, but most of all, John had watched Jesus die on the Cross only to rise from the dead. Yes, John had the credibility to be believed only a few could claim, and he wanted to remind the Church of it.