Students of Christianity, especially its earliest phases, have often wrestled with two problems, among many others:
(1) Why did this religious movement develop a fixation on dogma, on defining true and false (quasi) factual statements on a large range of subjects and visiting sometimes very harsh penalties on holders of the "false" beliefs, which is very rare among the world's religions?
(2) Why did it, on the one hand, preach a principle of love for humankind, forgiveness of faults, openness to all nations and races, and so on, but on the other, drive seemingly single-mindedly toward capturing the power of the state of the Roman Empire, and once having grasped this power, use it to wage warfare and establish a tradition of "just wars" which has continued up to the present?
I believe that there is an answer to both of these questions at once, which is the following. Any society defines a distinction between in- and out-groups in one way or another, since it needs its members to behave in certain ways which may not coincide with the behaviors expected by other societies. And since the members of any society may be tempted to violate their society's norms, there need to be sanctions of some sort which try to deter these violations.
In the case of the burgeoning movement of Jesus followers, it very early established the growth principle of trying to draw in members from the whole very complex population of the Roman Empire, with its enormous variety of traditional nations and cultures, rather than confining itself to being simply another Judaic sect. Paul took the development of the movement in that direction with his famous declaration that "there is neither Jew nor Greek, slave nor free, male nor female, for you are all one in Christ Jesus" (Galatians 3:28), which he repeated in numerous other forms, for example, "this righteousness from God comes through faith in Jesus Christ to all who believe; there is no difference" (Romans 3:22) and "Is God the God of Jews only? Is he not the God of Gentiles too? Yes, of Gentiles too" (Romans 3:29).
In doing this, he tried to preserve a link with the Judaic tradition by insisting—quite controversially in the view of those Jesus followers who preferred to see themselves as traditional Jews—that true Jews, that is, true worshippers of the God of the Jews, were not followers of the "law" (nomos in Paul's Greek) or the Mosaic rules laid down in the Torah, but believers in the saving power of that God's presumed "son," Jesus. We can see clearly in many parts of the New Testament that this was a highly contentious issue in the movement; probably Paul was not the only leader who welcomed "gentiles" (goyim) into the fellowship as full members. In any case, it was not long at all before practically all of the new Christians were being drawn from non-Jewish ethnicities throughout the Empire. Most Jews apparently were quite satisfied with their own new synagogue- and rabbi-based religious development following the loss of the Roman-Jewish War and the destruction of the Temple, which had been the center of Judaism, in 70 CE, and didn't want to touch this new cult of the crucified rebel, around whom who knew what kinds of followers were flocking, with a ten-cubit-pole.
But this presented a problem with which successive generations of Christian thinkers and writers struggled, but which they were never able to clearly resolve. They stoutly maintained that their religion was not only a legitimate inheritor of the centuries of Jewish tradition (which entitled them to a certain amount of respect from the surrounding pagan culture, which always admired ancient religious traditions), but the one and only legitimate inheritor. At the same time, though, what about the fact that, after a few generations, there were practically no members of the religion with actual Jewish ancestry, and indeed, there was a growing amount of mutual distrust and suspicion between Jews and Christians?
The open arms and declarations of love toward all humankind in the founding documents of the movement couldn't be completely repudiated; they were crucial to the spread of Christianity in the Empire, and its growing prestige and influence among the more powerful classes. (Without that support from at least some powerful persons, the persecutions which are so famous would have been much more continuous, widespread, and bloody than they in fact were.) But how to maintain the fiction—and indeed it had become a fiction—that Christians were actually the "true Jews," which was also a part of the original principles of the movement which couldn't be abandoned?
The answer was hammered out (and "hammered out" is the right term, because there were often violent clashes among rival Christian groups as they were written and adopted) in the set of beliefs that were eventually codified in the Nicene, Apostles', Athanasian, and other Creeds. To belong to the "in-group," one absolutely had to subscribe to whichever creed was currently in force. You could be of any race, nationality, family background, etc., at all, but you had to believe. In this way, the unity that any society needs was achieved, but it was achieved in a rather striking way, which probably had no precedent in history: it was a unity of abstract, philosophical principles, expressed in pious words but enforced, when necessary, in the most brutal ways possible.
This is especially clear in the Athanasian Creed, which probably originated (according to the Wikipedia article on it) in the late fifth or early sixth century. It was never as authoritative as, for example, the Nicene Creed (adopted by the First Council of Nicaea in 325), but expressed the attitude of the Church authorities quite clearly:
"Whosoever will be saved, before all things it is necessary that he hold the Catholic Faith. Which Faith except every one do keep whole and undefiled, without doubt he shall perish everlastingly. And the Catholic Faith is this: That we worship one God in Trinity, and Trinity in Unity; neither confounding the Persons nor dividing the Essence. For there is one Person of the Father, another of the Son, and another of the Holy Ghost. … Furthermore it is necessary to everlasting salvation that he also believe faithfully the Incarnation of our Lord Jesus Christ. … who suffered for our salvation, descended into hell, rose again the third day from the dead. … He will come [from the right hand of the Father in heaven] to judge the quick and the dead."
And so on.
With such declarations of the true faith, the Christian world had a solution to the above-mentioned contradiction. Yes, any one at all could be a Christian: universal brotherhood and sisterhood! But the God at the center of the religion was proclaimed to be the good old God of the Jews, no doubt about that, so the connection with the Judaic heritage was maintained; the whole body of the Jewish scriptures (or most of it, at least) was even incorporated into the Christian Bible. (Large parts of the "New Testament" wouldn't make any sense without reference to the "Old Testament," after all.) Oh yes, there was the matter of shoe-horning two more "persons" into the "trinity" which was the remodeled form of the Judaic God. But it was argued that this was simply an implication of the age-old Jewish belief system which, for reasons not clearly explained, no one ever realized until the lifetime of Jesus. But certainly there were a multitude of "proof texts" in the Jewish scriptures which clearly foretold the coming of God's Son. And if anyone didn't believe it, well, "whosoever will be saved, before all things it is necessary that he hold the Catholic Faith."
This also answers, I believe, the questions I began with.
(1) Once this structure of theological propositions was constructed, it provided the only foundation for the unity of what became Christian society. To keep the society together, the authorities were convinced, it was necessary to maintain eternal vigilance over the purity of the people's allegiance to those propositions. Hence the relentless policing of "heresies." Eventually a single Christian society in Europe (aside from the somewhat separate society of the Orthodox believers to the East) broke into separate nations, which proceeded to fight fiercely over precisely how the Creed should be interpreted until they finally decided that this warfare was senseless and agreed to differ over theology, since the principle of nationhood was sufficient to provide the in-group/out-group mechanism that each of the new societies needed.
(2) Why has the religion of Peace and Love (as it sees itself) fought so many wars? For two reasons, I think. First, its determined clinging to abstract theological principles (which in the minds of the true believers were never abstract, actually, but matters of passionate conviction) which were believed to hold the key to "salvation" for souls in eternity made it necessary to convert the whole of the human race to Christianity, and justified force against those who refused to be converted, because what is a little pain here on earth compared with eternal life? And secondly, once nations developed out of the single European Christian community, the Church authorities had no trouble at all blessing the troops of their respective nations. Once again, there is a handy quote from the Apostle Paul: "Everyone must submit himself to the governing authorities, for there is no authority except that which God has established. The authorities that exist have been established by God" (Romans 13:1).