The idea of the anonymous
Christian was first developed by a
Jesuit priest named
Karl Rahner.
Rahner and other
theologians found that with the advent of modern travel and communications, many Christians were coming into contact with people from other religious traditions than they would have in the past. This posed a problem because many of the
laity,wondered, "Why can't my
Hindu or
Jewish or
Buddhist friend who is a
moral person receive
grace?"
In the past Christian dogma would have said that these friends of other religious traditions cannot receive grace, but Rahner decided to change things a bit, and that's where the idea of the Anonymous Christian comes in. Rahner felt that Jesus offers salvation to everyone who lives good lives, whether or not they choose to believe in the Immaculate Conception, the resurrection or any of it. These people who have earned grace by living good lives, he called Anonymous Christians because Jesus knew them even if they never sought Jesus themselves.
As you can expect, not all (or even many) Christians buy this idea. In fact, following Rahner's ideas to their logical ends, you are left more with a Christian ethic or philosophy than a religion.