In What's So Amazing about Grace, Philip Yancey recounts this story about C.S. Lewis:

During a British conference on comparative religions, experts from around the world debated what, if any, belief was unique to the Christian faith.

They began eliminating possibilities. Incarnation? Other religions had different versions of gods appearing in human form. Resurrection? Again, other religions had accounts of return from death.

The debate went on for some time until C.S. Lewis wandered into the room. "What's the rumpus about?" he asked, and heard in reply that his colleagues were discussing Christianity's unique contribution among world religions. Lewis responded, "Oh, that's easy. It's grace." After some discussion, the conferees had to agree.

The notion of God's love coming to us free of charge, no strings attached, seems to go against every instinct of humanity. The Buddhist eight-fold path, the Hindu doctrine of Karma, the Jewish covenant, and Muslim code of law—each of these offers a way to earn approval. Only Christianity dares to make God's love unconditional.