Creationism certainly is an essential tenet of Christian fundamentalism. An interesting (and otherwise moving) book to read in this regard is Father and Son by Edmund Gosse. Gosse, an English scholar who later became Librarian of the House of Lords, was born in the C19 into the Plymouth Brethren, of which his parents were among the early saints. His father was a natural scientist of some renown, and spent his life trying to prove scientifically that Darwin, whom he greatly respected, was mistaken. There are still fundies trying to do this, I know one and once knew another.

My only reason to separate out creationism was to point out that it came along at that time. It is an essential part, but it should also be considered a semi-autonomous sub-belief-system, one with its own tenets and massive literature that theorises about the gaps or the correct literal interpretation and tries to prove it (or at least disprove science), something that goes further than just declaring the Bible history (='biblical literalism'). Hence creationism doesn't apply to earlier Christians who were biblical literalists.

_{*Lunatic*, n.
One whose delusions are out of fashion.}