Last time I looked at the statistics most people in the United States self-identified as Christian. But is self-identification the criterion by which we ascertain the authenticity of someone's religion? For most people this would be enough, and anyone who questions it is judgmental and should mind his own business.
The question does come up quite often. But is there a need to know? Religion in America has become highly privatized. What people believe about things like this should be confined to their own mind and excluded from the public arena. Public consensus seems to favor leaving questions like this one out of public discourse.
A pluralistic worldview has made significant inroads into Christianity. Now there are as many different answers to our question as their are versions of Christianity. And this doesn't seem to matter all that much to most people who would self-identify as Christian. Yet the original author and founder of the distinctively biblical version of Christianity is not interested in pluralism or public consensus. Truth interests him. And he has spoken to the question quite definitively and, I might add, quite clearly.
A Christian is someone who has been transferred from the dominion of sin and death to the kingdom of righteousness and life--Christ's kingdom. The Bible speaks of men and women as being dead in transgressions and sin. It speaks of this as a former condition when defining who a Christian is. It is out of this condition that a true Christian is taken. A new condition is established that is characterized as "the new self" or the "new creation." Indeed, the Bible speaks of old things passing away and new things coming in their place. No longer dead in trespasses and sins, the Christian seeks to know Christ, seeks to be found in Christ with no other righteousness than what Christ himself and Christ alone provides. But this isn't all.
A Christian is someone who has been united to Christ in remarkable ways, none of which can ever happen as a result of human effort. The Christian is united to Christ in his life. This is how the righteousness that is acceptable to God is acquired by someone who has no righteousness at all. The sinner is united to Christ's life of holiness and perfect obedience to God. Jesus Christ alone lived a human life with the purity and goodness that God intended and required. The sinner's union with Jesus Christ is the only way to possess Christ's righteousness without which God's acceptance is impossible.
A Christian is someone who has been united to Christ in his death.
Something must happen to our sin.
It cannot simply be ignored or brushed aside as no big deal. Sin must die. The penalty for sin cannot be dismissed. In biblical terms the Christian declares, "I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me" (Galatians 2:20 in the New Testament). It is Christ who dies in the sinner's place. Christians are Christians because they they have been united to Christ in his death to sin. Christianity is entirely unappealing if it fails to speak the truth of humanity's perilous condition and how that condition is removed.
The Christian is someone who has been united to Christ in his resurrection. This is how new life comes to defeat the old life of sin. Christianity would be a pointless waste of faith apart from being united to Christ in his resurrection from the dead. This union, performed by God according to his own grace and not according to our merit, is the only way a sinner who trusts exclusively in Christ is transferred from death unto life everlasting.
A Christian is also someone who has been united to Christ in his ascension to the Father's right hand--the place of highest honor in heaven. It is in that place where the Lord Jesus Christ now represents the Christian, intercedes for the Christian, and keeps the Christian's life "hidden in God" (see Colossians 3:1-4 in the New Testament).
The Christian is someone who is also united to Christ in his (second) appearing. The Christian is given the promise that he or she will appear with him in glory when he appears (see again Colossians 3:4).
A true Christian is a sinner who has trusted in these truths that pertain to Jesus Christ. No, I must go further: a Christian is a sinner who trusts exclusively in Christ. The Christian has forsaken all others to be united to Christ alone and Christ forever.
The sinner is brought to Jesus Christ in the hearing of his word (see Romans 10:17 in the New Testament). When the sinner hears the word of Christ and believes it, and puts his or her trust in Christ alone because of it, that sinner is made a Christian. The Christian lives all of life in joyful submission to Christ's word. The Christian loves the word of Christ, hungers and thirsts for the word of Christ, attends to the word of Christ with joyful obedience to the word of Christ (which is the Bible). Perhaps the most objective way to tell if someone is a Christian (or to tell if you are a true Christian) is to examine the heart for the role that the holy Scriptures play in the formation of one's worldview.
The Christian is someone who has also been united to other Christians who have been united to Christ. The assembly of these persons is called the church. The Christian loves the church because Christ loves his church. The church is not a place on a map. It is a people who have been made the people of God by his own choosing of them. There is no genuine Christianity outside Christ's church. There is no acceptable worship outside Christ's church. There is nothing but spiritual death and final condemnation outside Christ's church.
This does, of course, cut to the heart of pluralistically conditioned people. Within the boundaries of God's forbearance, people are permitted to deny and disregard the truth with all the venom they can spit. God's forbearance has lasted two millennia now. But God's word will endure. It will never change what it firmly, authoritatively, and clearly declares. God will never amend his definition of what a Christian is.
The Jesus Christ of the Bible sits now at his Father's right hand. At an hour known only to the Father, he will send his Son, the Lord Jesus Christ, for a second time to this earth to consummate his salvation in the glorification of his church and the creation of the new heaven and the new earth wherein only righteousness dwells. There will be billions of self-identified Christians confessing that Jesus Christ is Lord (see Philippians , but too late to have admittance to his presence. Having not been found in union with the Lord Jesus Christ, they will depart into unending conscious punishment for their sins. At that point, the question, How do you tell if someone is a Christian? will become irrelevant. Perhaps while it is still relevant, you will investigate for yourself how Jesus answers it in his word.