Christianity took from its great competitor a doctrine that may bring you to despair when you study the history of trinitarian and christological thought. The dogmatic development of Christianity cannot be understood without it.
Logos means "word". But it also refers to the meaning of a word, the reasonable structure which is indicated by a word. Therefore, Logos can also mean the universal law of reality. This is what Heraclitus meant by it ... The Logos for him was the law which determines the movements of all reality.
For the Stoics the Logos was the divine power which is present in everything that is. There are three aspects to it, all of which become extremely important in the later development. The first is the law of nature. The Logos is the principle according to which all natural things move. It is the divine seed, the creative power, which makes anything what it is. And it is the creative power of movement of all things. Secondly, Logos means the moral law. With Immanuel Kant we could call this the "practical reason", the law which is innate in every human being when he accepts himself as a personality, with the dignity and greatness of a person. When we see the term "natural law" in classical books, we should not think of physical laws, but of moral laws. For example, when we speak of the "rights of man" as embodied in the American Constitution, we are speaking of natural law.
Thirdly, Logos also means man's ability to recognize reality; we could call it "theoretical reason". It is man's ability to reason. Because man has the Logos in himself, he can discover it in nature and history. From this it follows for Stoicism that the man who is determined by the natural law, the Logos, is the logikos, the wise man. But the Stoics were not optimists. They did not believe that everybody was a wise man. Perhaps there were only a few who ever reached this ideal. All the others were either fools or stood somewhere between the wise and the foolish. So Stoicism held a basic pessimism about the majority of human beings.
Originally the Stoics were Greeks; later they were Romans. Some of the most famous Stoics were Roman emperors ... They applied the concept of the Logos to the political situation for which they were responsible. The meaning of the natural law was that every man participates in reason by virtue of the fact that he is a human being. From this basis they derived laws far superior to many that we find in the Christian Middle Ages. They gave universal citizenship to every human being because everyone potentially participates in reason. Of course, they did not believe that people were actually reasonable, but they presupposed that through education they could become so. Granting Roman citizenship to all citizens of the conquered nations was a tremendous equalizing step. Women, slaves, and children, who were regarded as inferior beings under the old Roman law, became equalized by the laws of the Roman emperors. This was not done by Christianity but by the Stoics, who derived this idea from their belief in the universal Logos in which everyone participates. (Of course, Christianity holds the same idea on a different basis: all human beings are the children of God the Father.) Thus the Stoics conceived of the idea of a state embracing the whole world, based on the common rationality of everybody. This was something which Christianity could take up and develop. The difference was that the Stoics did not have the concept of sin. They had the concept of foolishness, but not sin. Therefore, salvation in Stoicism is a salvation through reaching wisdom. In Christianity salvation is brought about by divine grace. These two approaches are in conflict with each other to the present day.
Paul Tillich, A History of Christian Thought (1968: 7-8)