"I don't care."
Such a response is common from evangelicals when presented with the history of Christian doctrine.
"Contraception has always been considered immoral, even by Protestants until the 20th century."
"I don't care. I think people were wrong. I don't see it in the Bible."
This means that it's possible that all Christians from the beginning were wrong on an issue of central importance (such as theology of marriage) until Christians were enlightened to the truth in the 20th century and finally discovered the true meaning of Scripture.
Sadly, this mentality has several serious problems:
First, it shows pride, sometimes even a hero mentality.
But an even deeper problem is that it tends toward turning Christianity into a discovered religion rather than a revealed religion.
Some religions are claimed to be discovered by man. People might meditate and discover truth about the universe. This knowledge is then pooled together in some way. New information is accepted, and the possibility of someone having been wrong is open.
Other religions are claimed to be revealed. Christianity is one such religion. A person meditating on a mountain did not figure out that God is a Trinity and then tell the rest of us. God, by his own initiative, revealed this truth about himself to us. The person of Jesus is the pinnacle and fullness of God's revelation to man. He is the eternal Word of God. (John 1.1)
When one dismisses Christian history entirely and is willing to believe a completely new doctrine that actually runs against the tradition, one begins to lean heavily on one's own ability, or at least one's own generation's ability, to discover new theological truth that was until then unknown. It must mean that God is still revealing, changing his mind, or that the Holy Spirit is so inactive in protecting God's supposedy definitive revelation in Jesus Christ that we need it revealed again.
Now, if history doesn't matter, what isn't up for grabs? In other words, if one is able to dismiss 20 centuries of teaching on one matter of theology, is there any piece of historic Christianity that one is not willing to dismiss? The Trinity? The dual nature of Jesus? Christ's atonement on the cross? Why? Who decides what makes the cut and what doesn't?
As a result, the word Christian starts to lose meaning. If truly everything is up for grabs, how do we define what Christianity even is? It becomes whatever a person who claims the title Christian wants to make it.
We are finite beings created by God.
We learn about God from God.
The work of theology is to better understand the revelation that God has given us, not to reinvent Christianity. When one claims that one has discovered the true meaning of Scripture on a particular issue that until then had not only been hidden but had been interpreted in the opposite way, it becomes difficult to justify such a view in light of God's history of revelation, which culminated and is completed in Jesus Christ.