The Christological argument for the existence of God is a relatively modern argument. It is an indirect argument based on the claims of Jesus Christ. That is, if one accepts that Jesus existed, that the Biblical account of Jesus is largely true, that Jesus' claims are valid, and that Jesus claims God exists, one should accept God exists.

Modern evangelism often takes this approach. Potential converts are introduced to Jesus as a historical character and the merits of Jesus's teachings are discussed. In such a context, the historicity of Jesus of Nazareth naturally takes on enhanced urgency; the usual historian's questions of documentation, authentication, and the like, tend to be removed from ordinary historical discourse, to take supportive places within Christological theology.

A related line of evangelical argument, associated with the apologist C. S. Lewis, and more recently with Josh McDowell and his popular series of books from his series that begins with Evidence that Demands a Verdict. This approach begins with the notion that people are willing to accept Jesus Christ as a philosopher and ethicist, but not as the Saviour, Messiah, or Son of God. It points out that Jesus is reported in the canonical Gospels as making those claims. It then attempts to recast questions about Jesus' identity to argue that since Jesus made those claims, either they were true and Jesus was in fact divine, or else he was a charlatan or a madman. For this reason, it is sometimes called the Lord, liar, or lunatic argument, sometimes called the "Trilemma," and sometimes called the "Lewis Triumvirate".

The argument need not be made as an argument for God's existence. It is also possible to assume (or separately try to establish) God's existence, whereupon one uses this argument to try to show that Jesus is indeed the God whose existence one has already established (or assumed). This is how the argument is in fact used by C. S. Lewis, who in his book Mere Christianity first argues, on moral grounds, that there is a God, and subsequently argues for Christ's divinity in this way.

This sort of argument is often criticized as a false dilemma; thus it may be said that this ignores, for instance, the possibility that Jesus was a moral philosopher, but that his reported teachings have been distorted or misrepresented in order to bolster claims of divinity. This sort of dismissal may be a bit too quick, however, since, in the first place, empirical reasoning generally does not present evidence so strong as to exclude all theoretically possible alternatives to the hypothesis in question (even the best scientific theories are not supported by evidence of such strength, let alone most well-substantiated historical claims), and, secondly, there is no reason, in principle, why the argument cannot be added to historical evidence which rules out other hypotheses besides the three considered in the argument. And defenders will support the Trilemma by arguing that Jesus's claims have been accurately reported. [1]

Interestingly, many of the tenets of this approach were under fire before it was even put to paper. Bertrand Russell, in his famous essay "Why I Am Not a Christian", criticized Jesus' personal character and philosophical positions on various grounds. However, Russell was accused of being rather ignorant of the actual claims of Christ. In any case, the argument is meant to exclude one commonly held view: that Jesus was merely a good moral teacher.

See: Arguments for the existence of God — Christology