There are Christians in the world, and there are non-Christians. Both groups can be further subdivided, but I am simply going to address this binary (only 2 options) situation.
Ultimately, as we read in Mark 16:16, “whoever believes and is baptized will be saved, but whoever does not believe will be condemned.” The world is separated into believers and unbelievers. The same outcome is illustrated in the parable of the rich man and Lazarus (Luke 16:19-31). At the end, every person either has saving faith in Jesus Christ . . . or not.
So, what makes a Christian? Is it a person’s dedication to the truth? Obeying the commandments? Church attendance record? Giving to others? Serving others? Being kind to others? Speaking the correct way (neither using inappropriate language nor putting people down)? Is there some minimum which a person must do to be saved?
A person is not a Christian because of what he/she does or does not do. Christian identity is God’s gift. Paul wrote to the Galatians that in Holy Baptism, you are clothed with Christ (Galatians 3:27). Though the Greek can be translated in the active voice (“[they] have put on Christ”), it is not something that you can seek or do. Luther explains in the explanation to the third article of the creed, “I cannot by my own reason or strength believe in Jesus Christ my Lord, or come to Him, but the Holy Spirit has called me by the Gospel . . .”
You are a Christian because you have faith in Christ, and that only because God has gifted you with that faith. Faith is not a quality that can be measured by us (Jesus can talk about “great faith” or “little faith”). All we can do is cling to the One who has called us and given us the gift of Jesus’ righteousness.
So being a Christian is not about what you do or do not do. It is about trusting in the One who has called you, and not turning away to false gods. As soon as you look inward and try to measure your amount of trust, you begin to lose that trust. Keep your eyes on Jesus, and He will preserve you until the end. That is His promise.