In chapter 3 of The Epistle to the Philippians Paul gives us one of the best definitions of a Christian available in the Bible. He also contrasts this with the marks of false teachers.
Paul begins the chapter by contrasting the wondrous gift of grace against the hopeless pit of sin. He warns the Philippians against false teachers; those, he says, who have confidence in themselves. That is, anyone who adds conditions for salvation, in addition to the grace of our Lord Jesus Christ. Paul likens to Gentile dogs, those who fail to recognize that salvation is wholly of Jesus.
But then in stark contrast to false teaching, Paul defines what a Christian looks like:
“For we are the circumcision, who worship by the Spirit of God and glory in Christ Jesus and put no confidence in the flesh”
He calls true Christians “the real circumcision”, i.e. the true covenant people of God. Then he gives three characteristics of Christians in verse 3. He says true Christians are those who:
1) Worship in the Spirit of God
2) Glory in Christ
3) And put no confidence in the flesh
(1) The first mark of a Christian is that they are those who worship in the Spirit. They are the true circumcision, Paul says. They do not worship in the flesh. The “flesh” here is not referring to our physical bodies, for there is nothing inherently wrong with physicality. God created all matter and our bodies and declared them “good”. What Paul is contrasting is human effort or trust in ancestry, that is, trusting in it for our redemption. In Scripture, “In the flesh” is always set in contrast to “in the Spirit”. They define two states of being or nature – those with the Spirit (regenerate) and those without the Spirit (unregenerate). “Those who are in the flesh cannot please God” (Rom 8:8) and “… the flesh counts for nothing.”… But “the Spirit gives life.” (John 6:63). And “no one can enter the kingdom of God unless he is born of … the Spirit (John 3:6; Ezek 36:25-27). Worship in the Spirit of God also means that the source of our daily spiritual life and walk in Christ is the Holy Spirit who unites us to Jesus Christ. Gal 5:25 likewise says, “If we live by the Spirit, let us also walk by the Spirit.” Thus both our conversion and our sanctification can be attributed to the work of Christ, applied by the Spirit.