It is probably impossible to define what it is to be a Christian in a way which would satisfy all those who sincerely believe themselves to be Christians. However, according to a widely accepted definition a Christian is a person who believes that Jesus of Nazareth is the Messiah or Christ and the son of God, and accepts Jesus' teaching as contained in the four Gospels of the New Testament.
As has been implied, this definition will almost certainly be challenged by some of those who regard themselves as Christians.
On the one hand, some may object that it is too broad or inclusive, i.e., it includes within the class of Christians individuals who should not be included. For example, Evangelical Christians, who believe that a prerequisite for salvation is a distinctive 'conversion experience', in which a person recognises himself or herself as a sinner and accepts Jesus as saviour, may insist that a reference to this 'conversion experience' should be included in any adequate definition, and maintain that those who have not had an experience of this kind, even though they may meet the conditions set out in our definition, are not 'true Christians' or are not 'really Christians'. Other groups of Christians may see our definition as too inclusive for other reasons: Christians have interpreted or developed Jesus' teaching in different ways, taking different elements in it to be central or crucial. Any group for whom a particular belief or practice is an essential element of Christianity (e.g., the doctrine of the Trinity) may well regard those who disagree with them on this point as not 'true Christians'. (See further Principal Christian Denominations and Heresy.)
On the other hand, some who regard themselves as Christians may object that our definition is too narrow or exclusive, i.e., it excludes from the class of Christians individuals (often themselves) who should be included. For example, Unitarians and Quakers, who do not necessarily believe that Jesus is the son of God but accord him special respect as an exemplary human being, may nonetheless see themselves, and be seen by others, as Christians.
The concepts of Christianity and Christian are examples of what the philosopher W.B. Gallie called 'essentially contested concepts'. Gallie introduced the idea of 'essentially contested concepts' in the context of political philosophy (Proceedings of the Aristotelian Society lxi, 1955, pp. 167-198), arguing that it is impossible to provide a 'neutral' definition of some central political concepts such as justice and liberty because those who subscribe to different political ideologies (e.g., Marxism, conservatism, socialism) will seek to define these concepts in a way that ensures they apply (possibly uniquely) to their own ideals. (See further Essentially contested concepts.)
The adjective 'Christian' is also used more widely and less controversially to describe anything which is related to, connected with, or derived from Christianity. It is in this sense that we speak of Christian art, Christian values, Christian humility, and even Christian agnosticism. Clearly a person may have Christian moral values, i.e., the moral values a Christian typically has, or should have, even though he is not a Christian; and a painter may produce Christian art without himself being a Christian.
See also Christian name.