To survey Christianity in the 20th century is far beyond what I am capable of doing (without becoming really boring). However, I think from 1950 on you can broadly see two big movements. From about 1950 to 1980, you have big clashes within Evangelical Protestantism between conservatives and "moderates" (really, they were soft liberals, but I am tipping my hand am I not?). The fights were nasty, splitting whole denominations. Lingering effects of it are still seen in the Mainline and Evangelical denominations today. Anyway, the clash produced very vitriolic debates centering wholly on theology and almost none on methodology. That's really where we get our image of angry preachers in old, hymn-driven, dead, formalized orthodoxy.
Spawned from that was the "Seeker Sensitive Movement" (1970s-2000s), pioneered by guys like Bill Hybels, the Willow Creek Association, and Rick Warren. They figured that the thing that kept people out of church was all that angry theological rhetoric and dead formality. Thus, they dressed down their suits to polos and khakis, starting bringing donuts and coffee into the sanctuary, and let dust collect on the pipe organ and piano. If theology and theological terms (verbal plenary inspiration, propitiation, inerrancy, hypostatic union, penal substitutionary atonement) turned people off from church and turned the cranky contenders on, then let's just ditch it. Instead, offer positive and practical sermons: "Five Ways to Improve Your Marriage", "Six Steps to Successful Stewardship", "Three Purposes of Fulfilling Family Time". Christian principles are in, "Christianese" is out. And it worked... for a time.
My parents' generation lived in the last days of the Judeo-Christian mindset. As Tim Keller put it, they have the ghosts of Christian ideas haunting their memories. They think like Christians without really meaning to. Tell them they need to pray that prayer and accept Jesus as Savior and they hear you loud and clear. They know what that means (more or less). Just don't turn them off with the fire of a divisive topic. The Bible, after all, is mostly about love they say. Emphasize that.
The Seeker Sensitive Movement of our parents has spawned two very distinct responses among us Millennials, starting in the 1990s and persisting into the present. The first to generate buzz was the Emergent Church (1990s-present). They took the seeker-hungry impulse of the Seeker Sensitive Movement to another level that their parents wouldn't. The parents saw theological language as keeping people out of church; the sons saw the theology itself as odious. So they figured, "Why not do to our beliefs what we did with our methods?" If organs can go, let's take out the divine child abuse of God's wrath with it. Protestantism has lost its authoritative place in American culture; maybe it deserved to be lost. Look at how bloody and vengeful traditional Christian theology has been. Scrap it then; let's save souls from the injustices of today. Rethink the very foundation, damn the exclusive beliefs, and maybe we can finally remake a Christianity that the modern world can respect again.
The other answer was the Reformed Resurgence (1990s-present). They grew up in Seeker Sensitive churches and found the doctrine shallow and wanting. They disliked the dead orthodoxy of their grandparents and could find no depth to it either. So they dug a bit further back, into the deep reservoirs of the Reformation. There they found the purpose behind the why of what we do and believe. There they rediscovered the language of the atonement; Jesus did not die on the Cross so that you could live your best life now. He died on the Cross as a propitiation for your sins. "Propitiation" means that God the Father puts His wrath on His Son and turns the wrath that was on you into favor. God's love for you is not merely the warm sentiment of a gentle father who smiles a lot; it is first and foremost the beaten and bloodied God the Son who took on finite human flesh and bled His way unto the Hill of Calvary and died an ignominious, criminal's death. All this was so that you might believe on Him and be saved from God's justice. Only a robust, deeply theological Gospel with lots of definitions and hard language could fully explain our faith to a generation of Millenials who have been born without the ghosts of the Christian worldview haunting them.
So what's all that got to do with anything?
All of us know or have known a cruel Calvinist. This guy (almost always twentysomething male) reads Desiring God or What is Reformed Theology? and decides that every other theological position denigrates the glory of God. Thus, everything is a fight. He has more evangelistic fervor for defending John Calvin than he does for Jesus Christ. He gets a reputation for being a jerk, a repute which totally figures because he is a jerk. He can express the soteriological nuances between Martin Luther and Theodore Beza, but he can't understand why women won't date him. He gets like that because he's a little drunk on deep truth and is angrily wondering why he was so long deprived by his parents' church. His solution to the wrong-minded, "KISS" (Keep It Simple, Stupid) church he is at is to sin against his brothers and sister to spite them. It's like the trendy piercing you got in college that your parents hated, except if you then responded to their hatred with increasingly gaudy and offensive studs until your skin got infected and no one would befriend you.
As I have gone through the "Why I Am Reformed" series, I know some readers have the cruel Calvinist in mind. It is tempting to apologize for that guy and condemn the bad name he has brought on Calvin. But as C.S. Lewis wrote in "The Dangers of National Repentance", it can be easy to say you're sorry for stuff that you never really did. It is really a form of criticism concealed under false humility; "I am sorry for all those guys who aren't as good as me at all." So I won't apologize for cruel Calvinists for two reasons: 1) I'm not one and they owe me an apology too 2) I am not John Calvin's PR rep anyway.
Let me offer something of real substance instead. I apologize to any who may have read my writing as vitriolic or who may have found me argumentative in real life. I don't want to start fires where Scripture is silent. If a wildfire is sparked because of something God said, then may His holy fire consume. But if it was me playing with matches, then I repent of being a jackass. I believe in a Reformed understanding of salvation because I think it is most biblical. I also know that there are no verses that match the Westminster Confession of Faith word-for-word; there's wiggle room on this issue and many of my most beloved Christian brethren are Arminians. Moreover, I confess and continually repent for being unfaithful to even clearer Biblical teaching than this. I have fallen prey to greed, lust, worry, covetousness, selfishness, and pride and battled them with less passion than I have advocated for secondary or even tertiary issues of interpretation. Sin is sin; simply being human doesn't make it less wicked.
I am Reformed because I believe only supernatural work of the Holy Spirit can bring me to a recognition of my own sin. Conviction and repentance are raw, soul-shattering, gracious supernatural gifts that I don't deserve.