The Apostle Paul gives us four identifying marks of a true Christian in the following...

Philippians 3:3 – For we are the circumcision, which worship God in the spirit, and rejoice in Christ Jesus, and have no confidence in the flesh.

1. We are the circumcision – God's elect who have been born again by the Spirit of God (regenerated and converted), brought to faith in Christ and repentance of dead works and former idolatry.
2. Which worship God in the spirit – We worship the true and living God, not according to our imaginations, but as He reveals Himself in His Word (The Gospel) as the God who justifies the ungodly based on the righteousness (the merits of the obedience and death) of His Son, the Lord Jesus Christ.
3. And rejoice in Christ Jesus – We see our whole salvation secure in Christ, based on His blood and righteousness alone.  We see that God freely gives us all the benefits and blessings of salvation and that we are fully entitled to the whole inheritance of grace and life eternal based solely upon the righteousness of Christ freely imputed as it is revealed to us by God-given faith.
4. And have no confidence in the flesh – We expect no blessing from God, no part of the inheritance of grace, and no part of our entitlement to Heaven to be based on our best deeds (nothing done by us or in us).  We see ourselves as completely saved and fully assured of all of salvation based on the righteousness of Christ before we take the first step in serving the Lord or persevering in the faith.