The Cross of Fire — Barry N. Malzberg

In the far future, it has become possible in advanced psychotherapy for a man to be given dreams as vivid as reality in which he may play any part he chooses. If that man were inclined to see his life as a struggle between good and evil, and if he were blessed with a profound sense of the black humour inherent in his situation, he might choose to play the part of Jesus, called the Christ. If he were inclined to write a book, it might be this one.