One of my problems with the Christian faith is that in every generation it reinvents itself to face the challenges of each generation. One would think that if a faith that was "once and for all delivered unto the saints" (Jude 3) this wouldn't be the case.
Christian theology has changed so much that one would not even recognize the Christianities of the first century or two.
Let me just mention some theological changes:
Creation - Not until around 200-700 AD did the church accept creation ex-nihilo.
Hell - From fire and brimstone to the absence of God to annihilation
Baptism - Probably from Immersion to sprinkling; from adults to infants.
Atonement - From ransom to satisfaction to penal-substitutionary to moral influence to relationship theories
Predestination - Possibly "mixed" to Calvinism to Arminianism to Calvinism back to Arminianism
Christology - From Paul to Chalcedon to Kenotic theories
Inspiration - From who knows what to mechanical to verbal-plenary to inerrancy to neo-orthodoxy.
Women - From servants who obey in quietness to teachers and ministers and professors
Slavery - From Paul (Philemon) to southern Slavery to abolition to anti-racism.
There are other theological ideas not mentioned, and there are many details to fill in about these particular ideas, but I dare say if today's Christians went back in time to first century Christianities they would not fit in, nor would they be considered orthodox in most Christian communities. Most all Christians today would actually reject the Christianities of the past. They would reject their theologies, their ethics, and perhaps their church polity (not mentioned previously). In fact, they would simply reject the Christianities of the past!
But what happened is that the church reinvented herself over and over in every generation in response to the challenges of her day. The church and her theology is like a chameleon to me in this respect. But the point is that it changed so slowly that few notice in today's world these drastic changes. Christians today wouldn't even admit that many of the earliest Christianites would be considered Christianity at all! And I know for a fact that the Christianites that will exist in the future will almost certainly be considered strange to the Christianites that exist right now.
I'm not looking for an explanation here, although I know what Christians will say. I'm simply pointing out that Christianity today is probably so far removed from the earliest Christianities that this is evidence that the church is a human not divine organization, one that flows downstream along with the history of human intellectual and cultural life. And I'm claiming this will continue to be true in the future too. At 54 years old I myself have seen these changes slowly take place inside the evangelical church for instance, with "Open Theism," the "Emergent Church," eschatological "Preterism," and an acceptance of babies out of wedlock, divorce in the church, abortion, homosexuality, and so forth.
So I ask, which Christianity is the correct one? There are a multitude of them which exist today all claiming to be the true church, but most all of the present ones would reject most all of the ones in the past, certainly the earliest ones.
Do church history. Do historical theology. It'll probably change your views about the church being a divine institution. It's not at all. The evidence suggests otherwise. Follow the evidence, okay?