The Gnostics were one of the first groups of heretics to detach from the Christian church. Their heresy stemmed from the fact that they believed that Jesus was all God, no man.
In modern times that seems like a silly thing to do - people are more apt today to believe the opposite! But the Gnostics were doing exactly what many modern folks are doing; they were trying to keep their Christianity culturally relevant.
Obviously, denying Jesus' manhood takes away His acceptability as the sacrifice for our own (very human) sin. But the Gnostics wanted to reconcile the teachings of Jesus with the popular Greek philosophy of their day that all matter is evil. God and His angels are perfect, but for God to enter earth as a human would corrupt Him; thus, they insisted that Jesus was fully God and never actually took on human form. In doing so, they rejected the Gospel.
A similar instance arose with early church theologians such as Augustine, who did not believe in a literal, six-day creation. Now, Augustine did not believe that the days were millions of years, as contemporary Christians may be inclined to believe! Instead - relying on an extra-biblical Jewish text - Augustine believed creation occurred in an instant. Again, He reinterpreted the Scriptures to fit this belief . . . because of Greek philosophies.
Martin Luther fought against this idea, as did John Calvin and the authors of the Westminster Confession of Faith. They all insisted that creation occurred as Moses described it, "in the space of six days." They did not care much about the predominant secular philosophies around them (as Greek philosophies were very popular throughout and even past the Middle Ages).
We have all heard the story of Galileo, the Christian man of Science who was persecuted by a church struggling to keep up with the times. The church was convinced that the earth was at the center of the solar system, and that the sun and planets all revolved around it. It is what the Bible teaches, so of course they threatened Galileo with excommunication if he did not forsake his heretical science that said the earth and other planets revolve around the sun.
Back up a second. The Bible says nothing about earth's place in the solar system. Where did the church leaders get that from? They got it from our old friends the Greeks, led by Ptolemy, who came up with this geocentric system! The church persecuted Galileo for his solid scientific research not because they were following the Bible, but because they were clinging to Greek ideas that had nothing to do with Scripture, placing them on a pedestal beside the Bible, and treating them as an equal authority.
Skip ahead to modern times. The predominant secular view about science and the age of the earth is that all living things have evolved over millions of years on a 4.5 billion year old earth in a 13.7 billion year old universe. This clearly goes against what the Scriptures teach, in that God created living things individually and "according to their kinds", made the earth before He made the rest of the solar system and universe, and did this all within the past six thousand years, in six individual days. Once again, there are Christians who do not want to believe this. They must keep up with the times. They must learn science like a well-educated, modern American intellectual and accept the secular philosophy of science that has allegedly been proven true. And so they (more often than not, unconsciously) twist Scripture's plain meaning to fit those views.
What if we did this in a more Gnostic style? Today, secular philosophies would say that there is no God - or if He does exist, He is not concerned with His creation, and would certainly never step into it as a man and meddle in people's lives. So 'Modern-Gnostics,' as I shall tentatively call them, would accept this secular philosophy and say that Jesus could not have been God, He was only a man. And yet, like the original Gnosticism, this would strip away Jesus' significance, and would destroy the Gospel message, for only God could live a sinless life and be an absolute authority on how we are to live our lives.
Of course, there are people who do believe this. We don't call them Christians, however, or Modern Gnostics. They are Agnostics*, the Pluralists or the Spiritualists of today, who claim that Jesus was a man and a good moral teacher. But such people are not Christians, just like the Gnostics were not Christians. But both groups were led into their beliefs because they were or are trying to mix the Bible with contemporary philosophies.
Most modern Christians can take one look at Gnostics and Agnostics and go, "Right. They are not Christians. Let's continue preaching the real Gospel message and move on." But when they look at Old Earth Creationism, at Christians who accept millions of years and mix it with Scripture, they don't think twice. They may accept it themselves. Nobody cares that early church theologians made the same mistake when they tried to keep up with the popular culture.
Now don't get me wrong; Old Earth Creationists and those early Christians who believed in an instantaneous creation are not dissenters of Christianity in the same way Gnostics and Agnostics are. They are, very often, pious and devoted Christians. But why do they feel as though they must do this? Why do we, as a church, see the popular cultural views of science and philosophy as so convincing, when our own history has shown that accepting them is folly, over and over again? What is so convincing about an old earth?
These grand ages for rocks and planets and solar systems are not pulled out of thin air, of course. They are determined largely through radioactive dating methods done on rocks. These methods are based on assumptions - assumptions that we know must be false.
Carbon-14 dating is based on the assumption that the amount of C-14 in the atmosphere has been the same in the past as it has always been. Now, we can use tree rings to determine how much C-14 was available in the year in which those trees were growing; we have done this, in fact, for trees up to 3,000 years old. And in looking at these ring-records, we know that the amount of C-14 in the atmosphere has varied over the past 3,000 years by as much as 70\%. And yet, these radioactive isotopes are used to date things supposed to be tens of thousands of years old, using the assumption that the amount of C-14 in the atmosphere today is the same amount of C-14 in the atmosphere tens of thousands of years ago. Obviously, this method is not going to be very accurate!
Then, for older dates, you have isotopes like Uranium-238, and various other ones like Rubidium, Potassium and Samarium. Not only have dating methods based on these radioisotopes given us million-year-dates to rocks we know are 50 years old (this was done with rocks made in the Mt. St. Helens eruption), but they have consistently shown themselves inconsistent with each other. They have even shown themselves inconsistent with C-14 dating. Scientists once found trees fossilized in rock beds and dated the trees and the rock they were found in separately; they discovered that the trees were a hundred thousand years old and that the rocks were several million.
Huh. I wonder how rocks harden, sit around for a couple million years, and suddenly get fossilized trees locked inside them.
Discovering problems in the 'science' and assumptions behind that 'science' in old-earth-evidence is not terribly difficult. Trying to understand why the church continues to rewrite parts of the Bible in an attempt to keep up with the times is. Especially when one considers how that has worked out in the past.
The problem is not science itself. It is historical science. The observational science that puts men on the moon is something everyone (or, almost everyone) can agree on; the historical science that says we evolved through the past millennia from apes is based on assumptions that everyone does not agree on at all. Believing in man's ultimate authority or God's ultimate authority lead to very different assumptions that lead to very different beliefs about origins, the past, and Jesus Christ Himself.
The truth of God's Word never changes. The alleged facts of man's word do all the time.
*When I say 'Agnostic' here, I mean it in comparison to Gnostic. Today, agnosticism refers to people who are unsure about their belief in God or who believe it is impossible to know whether or not God exists.