As far as I'm concerned, brushing aside theological differences and schools of thought, Christianity is the belief that Christ was the saviour of mankind or the equivalent in a different wording. As a religion, it's an extension of Judaism. Christians are people who adhere to the belief, but are also, by extension, Jews. Christians and 'proper' Jews, despite differences in belief, faith, doctrine and other details, generally maintain a certain sense of morality. Pragmatically speaking, a man who believes in no Christ but acts and thinks the same way as a Christian is a Christian.