As Christians, we are called to unity under our God multiple times, in multiple ways. In 1 Corinthians 1:10, for example, Paul exhorts us "Now I beseech you, brethren, by the name of our Lord Jesus Christ, that ye all speak the same thing, and that there be no divisions among you; but that ye be perfectly joined together in the same mind and in the same judgment."
Sadly, as is the nature of the human race, we have made a bit of a dog's dinner of this calling. There are many divisions within the Church today, and we even make some ridiculous excuses for why we don't all speak the same thing. Of course, sometimes, it is understandable that we have differing opinions. Scripture contains some very difficult information that we really need to sit down and discuss. The issue of predestination, for example. Scripture presents both a God who is fully sovereign over salvation and a human race who are fully responsible for how we respond to God's calls. This is a difficult issue, and so divisions are to be expected.
Nevertheless, the fact that we are called to have no divisions, and to speak the same thing, means we should at least try to do so. I strongly contend, however, that Old Earth Creationists are not pulling their weight.
For one thing, if we're all going to speak the same thing, it's not enough to speak it just for today. Rather, it should also be consistent with the beliefs of the Church in the first century and beyond. It is no good for the modern Church to be perfectly unanimous if we are in direct conflict with our original brethren. If, for example, we all collectively decided that bread is boring, and so we should break pizza for communion instead, our unity would be fruitless. We'd be wrong. Well, it turns out, Old Earth Creationism is in direct conflict with what was basically the unanimous belief of the early Church. For 2,000 years, "Young" Earth Creationism was the dominant position in the Christian faith, even standing in stark contrast to some secular origins stories that also presented an old earth. Old Earth theories were seen as mendacious heresies presented by pagans, and up until the 1700s, you would struggle to find either a Jew or a Christian who would disagree.
But of course, the flip side of this is that it's no good agreeing with the early Church if they differed from scripture. And we know they did, because a lot of scripture was written to them to say "hey, what are you doing? Stop that...". There are many ways in which scripture is in conflict with (our record of) the early Church. That's not to say the early Church was ever unanimous on some grievous error, of course. It would be hard to suggest, much less believe, that the men and women who served while the Disciples were still breathing would come to such ruin. Nevertheless, our imperfect historical record does record some very strange beliefs that were, or at least appear to have been held by at least a good portion of the Church. See, for example, this article.
So how do we know when we should agree with the early Church? Well, we could start by suggesting that if a view is non-existent throughout history, someone probably made it up. When it comes to differences of opinion on things like the Real Presence, or believers' baptism etc., we do see some very real debate. The early Church was not unanimous on whether the bread and wine are actually Christ's flesh or symbols thereof. Nor was infant baptism a universal practice. You find disagreement within historical sources on these issues, and so if you find yourself aligned with one of these people, at least you know your doctrine isn't something plucked from thin air.
Old Earth Creationism does not fit this category. It's not a case of one camp over here is YEC, one camp over there is OEC, and one camp over there thinks we're all nuts. No, there are literally zero suggestions in the historical Christian record that man could be descended from apes, that Adam may have been a metaphorical figure, that there may have been a pre-Adamic race, that Noah's flood was local etc. These are all ridiculous beliefs that exist entirely in our time. Even Augustine, who seems to be the one and only figure OECs appeal to, would have looked at their views and repeated "They are deceived, too, by those highly mendacious documents which profess to give the history of many thousand years, though, reckoning by the sacred writings, we find that not 6,000 years have yet passed." (1).
Now, as a general rule, if the only way you can ground your view in history is to misquote a man who thoroughly disagrees with you, you have no historical grounding, and so you have indeed plucked your doctrine out of thin air. But at the end of the day, I don't care how you interpret Augustine, because I have access to the writings of a far more competent theologian. Although it can be done, we do not need to prove that "Young" Earth Creationism is the dominant historical position. Instead, we have only to pick up this wonderful little book called "the Bible". This book, according to 2 Timothy 3:15-17, tells us everything we need to know about the Christian faith.
Now, once again, there are difficult issues within its pages. One can hardly expect to read a book inspired by God and have a perfect understanding of it. Nevertheless, the issue of origins is an easily accessible concept. Indeed, the reason the Church was so united on this one issue is precisely because it is so unbelievably obvious. The exact age of the earth may be a little more difficult to grasp if you're not great at math, but the actual historical details of creation are so easy to piece together, the only way to be an Old Earth Creationist is deliberately. It is not a conclusion reached by either reason or by accident, but rather it is born out of a frankly rather foolish desire to wed modern secularism, particularly atheism, with the Christian faith. Lacking belief in an old earth, no one would draw Old Earth conclusions from the Bible, and indeed no one ever did.
Now, if scripture is so clear on an issue, it should be very easy to unite on it. So here's the question: Why aren't we? Why do one particular group of Christians feel the need to deviate from the historical and Biblical view of origins? I simply do not understand it. I would assume it was pride or pressure had I not seen some very faithful servants of God fall for this nonsense. But I have. Christians I have a great amount of respect for have nevertheless needlessly conceded this ground to Satan. And my brethren, He is using it very effectively.
If we are to fulfil the Lord's vision for unity within His Church, we must fully unite under His word. Old Earth Creationism needs to be discarded 2,000 years ago, hung on the cross with the rest of our sins. May this folly cease to lead the sheep astray.
References
1. Augustine of Hippo - City of God and Christian Doctrine, Chapter 10.—Of the Falseness of the History Which Allots Many Thousand Years to the World’s Past