If you ask a theologian the question which forms the subject of this book: who is Satan? he will doubtless answer: Satan is the Commander-in-chief of the fallen angels. A Commander-in-chief implies an army, a hierarchy, a body of troops. What do we mean by angels? Are angels people like human beings, with a body and soul, or have they a more ethereal constitution than ours? Are they pure spirits, that is, without a body? In that case, what is meant by "fallen" in the theologians' definition of Satan? First of all, however, we must ask ourselves if here we are really concerned with authentic theology. Has the Church any teaching on the subject of the angels and Satan? Is the Christian, careful to believe everything that the Church believes and teaches in God's name, obliged to believe in the existence of the angels and Satan? Or is he allowed to consign the whole subject to the indeterminate domain of tradition, legend or Christian folklore? It would not be particularly surprising to encounter, even among enlightened Christians, those with doubts on the whole matter, people who prefer never to raise the question of the existence of angels and devils lest they be obliged to come to a decision about it.