In the paintings of the Catacombs the Good Shepherd willingly assumes the face of Hermes. But if their smile is the same, its import is quite different. Christian thought, which had to be expressed in a coherent system, thus tried to mould itself into the contours of Greek ideas and to express itself in the metaphysical formulas that it found fully constructed. But at least it transformed them. This is why, in order to understand what is original in Christianity, we must reveal the source of its profound meaning, and why, for historical purposes, we must return to its sources. That is the aim of this present work. But if research of any kind is to be coherent, it must follow one or two basic lines of thought. This introduction will help us to define these thought patterns, in so far as it will highlight certain constants which emerge, despite its complexity, in the historic material with which we are concerned.