What does initiation mean in the vocabulary of Christian theology ? Is this word the equivalent of the initiation rites of ancient religions, whose first concern is to insert young people into the reality of adult social life ? Christianity offers another vision of this issue : initiation opens one to a new life, the Kingdom of God, and this is accomplished through the sacraments. By affirming this, the Church provides a key for evaluating the specific relationship between the present world (« nature ») and the irruption of the new world (the « Kingdom of God ») : nature is the sign of grace. Rupture as much as newness, initiation establishes a relation of otherness between the two domains, as a sign is entirely oriented towards the otherness of the reality that it must designate.