John 14.15-21 (https://www.biblegateway.com NRSVA)
What is a Christian? This was an urgent question in John’s church because there were people who claimed to be followers of Jesus but who held very different views from John and his friends. John talks about them explicitly in his letters, and by the time these were written things were getting pretty tense. The church had split, with some members leaving to pursue a very different direction. The word ‘antichrist’ is banded about freely in the letters. It’s a term used solely by John, and only to describe his opponents. No one knows precisely what it means, but it’s not a compliment.
It was very important to John to be able to tell the difference between the real followers of Jesus Christ, and the people who - although they said they were Christians - were really ‘anti-Christs’ because they took a totally opposing view. John says that the true followers of Jesus are ‘they who have [his] commandments and keep them.’ These are the people who really love him. So being a Christian is about believing the right things, having Jesus’ commandments as our guiding principle, and then keeping them, or embodying them, in our daily life.
Some of the anti-christian teaching which John identifies in his letters seems to have been that Jesus didn’t really die on the cross in the same sense that we die, because his real, eternal self was separate from his mortal, physical body. But we already know that John can’t accept this, because he believes that the most important commandment of Jesus is to love one another in the same selfless way that he first loved us in dying for us on the cross.
Loving one another in the same unconditional way that Jesus loves us sounds like a very tall order, but John prefaces this definition of a Christian by saying that God will give us an advocate or helper to encourage and support us on our faith journey. Technically the term John uses means ‘the counsel for the defence’, but here he seems to be using it in a more general sense of ‘adviser’.
That this Spirit of Truth is the same Spirit that was in Jesus is made clear when he explains, ‘I will not leave you orphaned; I am coming to you.’ So a Christian is someone who is empowered to live in a Christ-like way by Jesus’ own Spirit at work within them.