Think for example about what Paul says in 1 Corinthians 13. Here is a church where certain gifts are being held up before the people as indications of a kind of super-spirituality. And basically what Paul says in 1 Corinthians 13 is, “You can have all of these gifts and actually be nothing in God’s kingdom.”
So what would be the sign? The sign would be the presence of faith and hope and love (1 Cor. 13:13): faith in our Lord Jesus Christ, trusting in Him; hope—remember how Peter speaks in 1 Peter about the hope that is given to us in Jesus Christ; and the third and most obvious one would be a love for Jesus Christ that produces in us, first of all, a love for His people and then a love for those who don’t know Him.
A good place to go and answer that question is in John’s first letter. He wrote the gospel in order that people would come to faith and keep on in faith, but he tells us that he wrote the first epistle to help people to understand what the marks of being a Christian are: that we trust in Christ, that we walk in the light, and that we walk in love. These are the three essentials.
Of course we grow within that, and it’s a bit like a tree, isn’t it? Branches grow out of that. But that’s a good place, I think, to begin. In the past that was often the place where Christians began their exposition of the question, “What in practical terms does it mean for me to be a Christian?”