I am certainly not the only one who has voiced deep suspicions regarding the idea of secular spaces and parts of life, but I have voiced them, here. On re-reading those posts, however, I realize that there may be cause for confusion, and so I wish to make a clarification: while I do not believe that I can divide my life into the religious and the non-religious, I will defend secularism as a political institution. That is, I will argue for the institutional separation of church and state.
I can understand, though, how a devoted Christian (or other believer) could desire a conflation of a religious group and the ruling party. If you do truly believe that your religion offers the best understanding of the universe and the best moral framework, then it may seem to follow that your religion should guide (dictate?) your society's political decisions. The best way of ensuring that, unless your electorate is homogenous, is to have your religion unambiguously in charge. (I think of C S Lewis' allegory of the fleet in Mere Christianity.)
There are some obvious problems to this: for instance, even if Christianity were in charge, I don't think it's clear whose Christianity is in charge. Pope Benedict's? Pope John Paul II's? (I'd say these are different.) Fred Phelp's? Gene Robinson's? Pat Robertson's? Alexis Seniantha's? And how would such religious-political power be enforced? This also holds true for any kind of ideological power, really: can you maintain it and enforce it without violating the principles of that ideology? The sort of Christianity I believe in can't be enforced without contradiction, and this is, I think, an important point. Again I would like to turn to Huston Smith on Hinduism (not that I get everything I know about religion from The World's Religions, but I find it's succinct and usually make explicit what I want to refer to).
In the following passage Smith explains the role of the brahmin in the Hindu caste system:
Members of this class must possess enough willpower to counter the egoism and seductions that distort perception. They command respect because others recognize both their own incapacity for such restraint and the truth of what the seer tells them. [...] But such vision is fragile; it yields sound discernments only when carefully protected. Needing leisure for unhurried reflection, the seer must be protected from overinvolvement in the day-to-day exigencies that clutter and cloud the mind [...]. Above all, this final caste must be protected from temporal power. India considered Plato's dream of the philosopher king unrealistic, and it is true that when brahmins assumed social power, they became corrupt. For temporal power subjects its wielder to pressures and temptations that to some extent refract judgment and distort it. The role of the seer is not to crack down but to counsel, not to drive but to guide. [emphasis mine]
We see here that it is not only political, institutional power that corrupts the brahmin, but also social power. It is likely an oversimplification to say that the worst excesses of Christian churches--whether Catholic, Orthodox, Anglican, Pentacostal--are a result of power, but I am certain that power was involved, and I think a cursory glance through your own understanding of history will support that.
For instance, much of my sense of the specifics of doctrinal history come from my Lutheran confirmation class some years ago. To my understanding, many of the problems that Luther sought to reform in the Catholic Church, leading to his excommunication and, eventually, the formation of one of the earliest Protestant churches, were fairly obviously instances in which the political power of the Church resulted in a corruption of its doctrine. In particular, the sale of indulgences could only exist in a setting where the church dealt in wealth and power. (Or so you'd think, but Scientology and the celebrity version of Kaballah do sometimes exchange spiritual status for money, and I'm not going to say that televangelists are above similar tricks. But these are somewhat different phenomena.) More than this, Luther was forced into exile because of his heterodoxy. When political power and religious groups are closely tied, heresy is necessarily political. (In better news, it wasn't much later that the Catholic Church conceded he was right on the indulgence count.) If you prefer other examples, I can think of the abuses performed both on Aboriginal bodies and on theology in Anglican missions in Canada, or the current theological "work" being done to support the subjugation of female and homosexual individuals, and to support aggression against Islam (in the past it would have been communism), in certain Protestant churches today.
And so I support the separation of church and state not just to protect the state, but to protect the church. The role of any religious institution is to guide and teach, to offer aid and sanctuary. The effect power has on these roles is largely negative. I had a professor once who said that Christianity might improve for its loss of cultural influence, by forcing it to remember its origin as an outcast belief and thereby forcing it to remember its duty and allegiance to the marginalized, an allegiance all but forgotten these days.
A word of caution, though: I imagine some of the ugliest forms of Christianity that exist today get their ugliness from their loss of power. That's not to say that the loss of power won't, in time, be cleansing for most, but for those invested too much in the idea that America is a Christian nation, or the equivalent of that in other countries (like my own), this loss of power will result in some atrocities. I want to note, though, that this is still a result of power's corruption; you just don't always see the rot until you kick the log over.
And another word of caution: The role of the brahmin is to give direction to society, including and perhaps especially the prince. Isolating the brahmin from power doesn't mean he or she has no role in it. The trick seems to be that the brahmin's resulting power must be invisible to him or her, the (hopefully healthy) inverse of an Ender's Game scenario.
And a final word of caution: It is not only religion that is corrupted by power. Economic or scientific beliefs, even when in themselves good, can be too. While Marx may have been on to something, Stalin wasn't; while genetics is true, when combined with power you might get eugenics.