It seems to me that best, most concise and objective definition for Christian is: someone who follows Christ. This seems rather obvious at first blush but hold on! Perhaps it doesn’t matter so much what we profess to believe (the devil himself could arguably affirm the Apostle’s Creed) but who we actually *follow*. It is possible there are those who follow Christ without even knowing his name. Abraham, Moses and the prophets of old might fall into this category. Even today, there are many Christians who don’t know Jesus’ true name (it’s not actually, ‘Jesus’, btw). Could it be possible that some people who call themselves Buddhists, Taoists or even Secular Humanists are actually following Christ whether they know it or not? That one is far beyond me but I definitely think it’s worth pondering. Jesus told an excellent story which I believe illustrates my point: Matthew 21:28-32.
What God said to Abraham was not 'Obey this law and I will bless you', but 'I will bless you; believe my promise'.