During the Age of Enlightenment, a theory made popular by a Scottish theologian named Thomas Chalmers. The Gap Theory was formed to help make sense of the supposed millions of years of Earth’s existence while also believing in the Bible. The Gap Theory proposes that in between Genesis 1:1 and Genesis 1:2 there are millions or billions of years of unrecorded history. During this time, there was an event called “Lucifer’s Flood” where Satan and 1/3 of the angels fell from heaven after an act of rebellion. During that flood, the waters caused catastrophic damage to the Earth thus resulting in the geological formations we see today. Also during this time, the dinosaurs were buried and fossilized in the sedimentary rock.
This theory helps explain why the Earth looks like it is millions of years old, but there is no biblical basis for the theory. Those who created it were adding to the Bible to accommodate for the leading old-earth paradigm at the time. The Christians did not want to look foolish for ignoring scientific theories, so the Gap Theory and others like Theistic Evolution and Progressive Creationism were created to accommodate for the secular theories. There are theological problems with believing in an old earth and death before sin.