Unlike Islam and Judaism, Christianity --like Buddhism--was not designed to be the foundation of a political order. The religions of Moses and Muhammad are based on Gods who specialize in the creation and promotion of legal codes for independent political communities. Buddhism is fundamentally a monastic religion, with householders as useful and necessary but second-tier members.
While Jesus preached the coming of the Kingdom of God, there is no evidence that he was at all interested in a religious state. Render unto Caesar, My kingdom is not of this world, etc. And if there is a form of Christianity that most matches the kind of life we see in his three-year career, I suspect it is more like the medieval mendicants than either Constantine's court or the monasteries of Benedict and Basil...or a parish church. Christianity, although it is founded upon a unique Incarnation of God and man, is not really at home in this world, this planet earth we know.
The Christian state was an accident of history (or a work of Providence), but an overwhelming reality which long outlasted the initial three hundred year epoch of non-establishment. While Christianity did not start out as either a political or a monastic community, it eventually became both. And you can argue that the rise of monasticism in the 4th century Egyptian deserts was a clear response to the increased respectability of new imperial faith that had for its first three centuries been both largely urban and either marginal to or actively persecuted by its surrounding culture.
Since (rabbinic) Judaism and (Sunni) Islam are householder religions, they are somewhat egalitarian. Experts in religious law --the rabbinate and the ulama-- hold positions of leadership and authority, but these are based on knowledge of tradition, not on alternative lifestyle or sacramental powers. In these faiths, all believers are held to the same standard and same way of living, more or less.
Buddhism is a monastic faith; at its heart are communities of celibate monks, withdrawn from ordinary life. If there is a legal code in Buddhism, it consists in monastic rules. Laymen are held to a much less stringent standard. (Gnosticisms often, though not always, likewise provided a two-tiered community: the perfect and the listeners.) IMHO, this is based on the recognition that you cannot live monastic life and a family life at the same time, that if you withdraw from the world, someone else has to run it.
When Christians rather suddenly found themselves not persecuted by the State, but in charge of it...well, it became (and continues to be) a bimillenial and often very complex and messy tension. I say "continues to be". Despite the disappearance of the Christian State, Christians, having had their faith shaped for a thousand and a half years by political power, are in the habit of bringing their religious convictions to bear on the realm of Caesar. Even in the current West, where the liberal state has emancipated itself from the Church while living off a distorted version of its morality (along with messes of pottage from the Enlightenment and Marx), one which has rebounded back onto the Mater Ecclesia.
My FB friend ER asks a very good question: "How can you pretend to love Jesus and yet not (abolish the separation of religion and politics in order to) demand that the Western state, military and economics obey Jesus' Beatitudes?"
One of the oddities of the social justice Christians, all of whom support the secular state, is that they then expect that state to act like the Community of the Beatitudes. Or when it does its secular state thing, acting like Caesar --the killing of Osama being the current issue-- they tsk-tsk and point out their own high moral code prevents them from being pleased at its actions. And as others have pointed out, social justice Christians expect the state to adopt the social benevolence and pacifist aspects of the Gospel, but --with some exceptions-- they find any post-Christian state that tries to enforce the Gospel's sexual code on marriage or divorce oppressive, tyrrannical and dogmatic.
One of the strategies of Christendom, both in the complexities of Church/State and householder/monastic, was the clear separation of realms or distinctions within the Gospel such as the precepts vs the counsels. One of the benefits of the older, and traditional view, that the celibate monastic life was superior to the householder life in the world was that men and women living an ordinary life, either as shopkeepers or soldiers, were also not held to a higher moral standard. That seems to me to have been a fair and humane trade. The current crop of crypto-Constantinians*, who want to have their Bread and eat it, too, simply confuse supernatural holiness with natural survival.
If you want to practice what you imagine to be a pure, evangelical Christ-like life, then keep your nose out of the polis and stick to the skete. Because it is "only because rough men stand ready to do violence on their behalf that these good people sleep in peace in their beds at night". Have at least a little gratitude, o ye saints.
In a world where Caesar has his necessary, even God-given, role (see Romans 13), these high-minded Christianists (!) who want a besieged Western civilization to act like monks or Mennonites are not worthy of respect. At least from me.
*The crazed idea of Protestant Dominianists like Rushdooney is, to me, beyond discussion.