The question of the age of the earth – and of the universe itself – is one of the areas in which there is perceived conflict between modern science and the Christian faith. In the pre-modern and early-modern periods, when the relevant science was not advanced enough to estimate the age of the earth, the prevailing view in the west was that the earth was roughly six thousand years old: a number pieced together from Old Testament genealogies. At the time, this biblical calculation was not in opposition to science. Isaac Newton – the greatest scientist of his age – was among those who calculated the age of the earth at 4000BC.
By the 19th century, however, science had advanced sufficiently to bring estimates of the age of the earth out of the thousands and into the millions. Lord Kelvin (aka William Thomason) was one of the pioneers of this field of study. He developed a view that the earth had once been a red-hot, molten globe and estimated its age on the basis of how long it would take to cool into a habitable state. In 1864, he estimated that the earth was between 20 and 400 million years old.
Kelvin was a very committed Christian. While his estimate of the age of the earth at 20-400 million years did not come close to the 4.54 billion years calculated by today’s science, it was a far cry from the 6000 years of biblical calculation. Kelvin’s estimate did not, however, allow enough time for Darwinian evolution to take place. This did not lead Kelvin to reject evolution entirely, but rather to adopt a view of theistic evolution within which God sped up the evolutionary process. [i] Kelvin had a public disagreement with geologist John Tyndall and Thomas Henry Huxley (Darwin’s bulldog) on this question. While Kelvin turns out to have been mistaken in his estimate, his reasoning was consistent with the best understanding of physics and energy sources of his day, and gives us an example of a committed Christian as far back as the 19th century arguing for an “old” earth.
In the early 20th century, however, the fundamentalist movement of Christianity became an influential source for the “young-earth creationist” perspective, according to which the earth is a few thousand years old. This led to many protestant Christians – particularly in America – holding to a young earth view that is not aligned with today’s scientific consensus. From a theistic perspective, the young-earth view is not irrational. Some advocates of young-earth creationism argue posit that God created the universe with the appearance of age, or that future scientific discoveries will overturn the current consensus. But it is also important to recognize that Christians have historically held different views about how to interpret the bible in the light of scientific discoveries and whether holding to a young-earth view is theologically necessary, or even indicated by the scriptures.
[i] Bowler, Peter J. (1983). The eclipse of Darwinism: anti-Darwinian evolution theories in the decades around 1900 (paperback ed.). Baltimore: Johns Hopkins University Press. pp. 23–24. ISBN 0-8018-4391-X.