What is a Christian?

The gospel of grace proclaims that all people everywhere are included in Christ. What then is the difference between a Christian (believer) and a non-Christian (non-believer)? Answering this question begins by noting what is common to all people:
  1. All are "in Christ."  The eternal Son of God, our Creator and Sustainer, became through his incarnation the vicarious (representative - substitutionary) human being.  Paul refers to Jesus as the second (last) Adam, who stands in for all people (Romans 5:12-21) and is the fountainhead of a new (re-created) humanity (2Cor 5:17). Because of what Jesus has done in union with all humanity, all people are now reconciled to God (Col 1:20, 2Cor 5:19a). 
  2. Jesus became human for us all; taking on our nature and thus becoming sin for us (2Cor 5:21). When he died it was thus all of sinful humanity that died with him, and all sin was atoned for through him and in him (Romans 6:10; 2Cor 5:14-15). 
  3. Then when Jesus rose from the grave to new (glorified) life, all humanity rose with and in him (Romans 5:18; 2Cor 5:17), and now all are "seated" (exalted) "in Christ" in the "heavenly realms" (Eph 2:6).
  4. Paul refers to this inclusion of all people in Jesus' life as our "adoption" (Eph 1:5). In and through Jesus the vicarious man, God recreated all people (2Cor 5:17), making them his dearly loved children (Eph 5:1). This is now the new identity of all and the new identity in which we are invited to live.
  5. By virtue of this inclusion, which means atonement, reconciliation and adoption for all, God has extended forgiveness to all. God no longer counts (keeps record of) sin against anyone (2Cor 5:19b); there is no condemnation emanating from God toward anyone (Romans 8:1).
Scripture proclaims these things to be true of all humans - those dead, those now living, and those yet to be born. These things apply to all because Jesus has included all with and in him; no exceptions. But note that there are further issues to be considered - ones that define important differences (distinctions) between believers and non-believers - including the following: 
  1. A believer is one who has come to know of their true identity as God's dearly loved, forgiven, adopted child (1John 3:1).  What was already true for them (and for all) objectively, is now becoming "real" (or realized) in their personal, conscious awareness and life experience. Believers are becoming what they truly are in Christ.
  2. This profound "awakening" to God's grace in Jesus is directed by the Holy Spirit, who gives each person (according to his perfect timing), the opportunity to hear the gospel of their inclusion (Heb 4:7, Romans 10:14); inviting them to change their thinking (repent) in order to align it with the truth (Acts 2:38); and to deeply trust (have faith) in Jesus (Romans 10:9) who is the truth, the way and the life (John 14:6).
  3. This hearing, believing and trusting is God's good work in the believer (Php 2:13) - their sharing in Jesus' perfect human hearing, believing and trusting for us and with us (but note that this sharing is never forced by the Spirit upon a person against their will). 
  4. Through this Spirit-enabled repentance and faith, the believer lays aside the alienation in their mind toward God (Col 1:21), allowing for a new, personal participation in the reconciliation they have always had with God in Christ, but heretofore did not personally know of and thus did not directly and actively experience (2Cor 5:20). 
  5. This personal, realized participation of the individual in their true life in Christ, leads over time to profound life transformation because they are now open to and actively sharing in the work of the Spirit who conforms them to the image of Jesus (Romans 12:2; Php 2:1-2, 5).  C.S. Lewis writes (in "Mere Christianity," p. 167) concerning this life transformation. His words give voice to Jesus' invitation to believers to go deeper with him:
"Give me All. I don't want so much of your time and so much of your money and so much of your work: I want You. I have not come to torment your natural self, but to kill it. No half - measures are any good. I don't want to cut off a branch here and a branch there. I want to have the whole tree down... Hand over the whole natural self, all the desires that you think innocent as well as the ones you think wicked - the whole outfit. I will give you a new self instead. In fact, I will give you Myself: my own will shall become yours."  
Jesus has already given himself for all and to all (Eph 4:6). All are included in his love and life. But the believer has come to know and believe in this inclusion, and has begun to live actively into it. In a response of faith, empowered by the Spirit, the believer gives themselves more and more (though imperfectly) to Jesus who they know to be their life and who makes, for them and with them, the perfect human response to the Father on their behalf.  

In this sharing in Jesus' perfecting life through the Spirit, the believer is gradually transformed from the inside out (2Cor 3:18). This is the experience and the identifying mark of those we refer to as "believers" and "Christians" - those referred to in Scripture as "saints."