You seem to be referring to both churches that one might attend on a Sunday, and church in the abstract, which transcends its material aspects both human (clergy, lay adherents) and inanimate (buildings, pews, hymnals) that come and go over the centuries even as the church as an abstract institution persists and evolves. The latter is a set of traditions, doctrine, stories, values, goals, history, and the like.
The church in the latter respect, which has its roots in early organized Judaism, arose to meet a variety of needs. The priests were already comforting people whose lives were short, brutish, and frequently uncertain. Living on the edge or in perpetual danger creates a need for religion, which might be why it appeals more to the poor in Africa and Asia, and the relatively disenfranchised segments of American life such as people of color and poor whites, than to comfortable, educated citizens. For the latter, for the church to be effective, it needs to reach people before the age of critical thinking, and create fears and a need in them that only it can solve.
The chief beneficiaries of an organized church are the clergy. It was an opportunity for selected individuals to hold positions of social eminence while earning a living without having to labor using one's back in the hot sun. And like all businesses, it became profitable to expand the operation to include more people per cleric. Ask Joel Osteen. Build it and they will come, and they will bring their wallets.
There's a clue that this may be correct in the Old Testament, but one has to go to two parts of it: The creation story and the Ten Commandments. The Lord took the seventh day to rest, and so must you. Honor the Sabbath, which is clearly one literal day.
This is really an odd idea. Why would a god need to rest, and even if it did, why would we need to imitate it and rest?
I have a hypothesis about the origin of these ideas that relates to the arising of a clerical caste and organized religion.
Go back a few millennia, before the advent of the week and the weekend, when people worked every day, and it was likely socially unacceptable for able bodied people not to work every day. Perhaps it was taught that the gods expected it. This was very likely true in man's nomadic days of hunting and gathering, and probably applied even when he settled into a farming and herding life.
Now, fast forward to the advent of monotheism, organized religion, temples, and a priesthood, which would like it to become necessary for every head of the household and probably everybody else as well to periodically come to the temple with shekels to sustain this activity. This might be several hours of travel in each direction with several hours of congregating, which meant taking time away from work.
How do we manufacture support for that idea that it is OK to take a day off if work is considered sacred and holy? Easy. Make taking a day off once a week even holier. In fact, make it a Commandment. Even the Lord rested on the seventh day, and you will, too. And that will be a day of worship. Your presence is expected under penalty of social stigma if not cries of apostasy or impiety.
This seems very plausible to me, since it accounts an otherwise inexplicable and counterproductive idea - that a god needed to work for six days, or needed to rest - by imputing a very familiar motive to the practice. I'm betting that that story of the six days of creation followed by one of rest was concocted to imitate the cycle in man's life that the priests were trying to establish and sanctify.
Another possible clue: Look at how artificial the week is. A day, a month, and a year are each natural units of time reflecting celestial events: one rotation of the earth about its axis, one revolution of the moon around the earth, and one revolution of the earth around the sun. Why do we have the week? Probably because the day is too short and the month and year too long an interval between trips to the temple.
And so, the week was created, and with it, the weekend, or Sabbath as it was originally known.
Another group that benefited from an organized church was the kings (and emperors and tsars), which is undoubtedly why Constantine promoted Christianity. It serves the interest of monarchs to teach the masses that the king rules because God has chosen him, that resisting him is defying God, and that the proper attitude for the ruled was to accept their lot however exploited without rising up. Thus subjects were expected to submit to kings, slaves to masters, and wives to husbands. Instead, they were instructed to be meek, told that they should be long-suffering, to turn the other cheek if smitten, accept poverty (your reward comes later), and the like. As Napoleon said,
- "How can you have order in a state without religion? For, when one man is dying of hunger near another who is ill of surfeit, he cannot resign himself to this difference unless there is an authority which declares 'God wills it thus.' Religion is excellent stuff for keeping common people quiet. Religion is what keeps the poor from murdering the rich."
Yes, I know that this is a cynical view, but it's rooted in human nature. Priests don't want to have to labor, kings want subjects to bend the knee, the basic disposition that worship of gods facilitates. And neither group wants ordinary people to read, think, or doubt - just believe, which is why faith is considered a virtue and skepticism a sin.
Even in a modern liberal, democratic state, the church still serves these purposes. The Republicans motivate believers to vote for them with promises of political rewards of interest to Christians, and it seems to work very well, especially with the cooperation of the church promoting such ideas and candidates. How else are we to account for the white evangelicals, for example, voting overwhelmingly for a man that defies most of their stated values if not in the hope that he will put justices on the Supreme Court to overrule their least favorite decisions from the past?
I hope that I addressed your question about what has become of the church and why it has evolved in the way it did.