Why Genesis chapter 1?
Genesis chapter 1 tends to be something of an embarrassment to Christians because of the quite false claim that it represents the earth as having been created in 7 periods of 24 hours. That is simplistic. In the Hebrew scriptures the word for "day" was from time to time used metaphorically (e.g. Genesis 31:40), just as it is in modern English. It can refer to any period of time. When old guys like me say: "In my day ... ", we are not referring to 24 hours -- more like decades.
There is however some cause for embarrassment if one knows what Genesis chapter 1 really is. I have forborne from mentioning it so far out of respect for my Christian readers but in the end I think it is important that knowledge buried in scholarly publications should be brought into public view. So I am now breaking my self-imposed embargo. Readers at this point may wish to decide if they should continue reading.
For a start, it is clear that chapter 1 (plus the first three verses in chapter 2) is a late tack-on, and a glaring one at that. It is the first of two different accounts of creation and has major textual differences from the original account given from Genesis 2:4 onward. The really glaring difference is the use of the divine name. In the rest of the Torah, the divine name (Yahveh; Jehovah) is used freely in the original Hebrew text. Eventually, however, pietism took hold and use of the divine name came to be regarded as disrespectful. "Elohim" (God) and "Adonay" (Lord) came to be used instead. We see something similar among modern Jews, where the usage "G-d" is now common.
So what do we see in Genesis 1? Complete avoidance of the divine name. And from chapter 2 onwards the name is used freely. So chapter 1 is clearly from a later era.
But what could have motivated something as serious as a distortion of the original creation account? Sun worship. It was an attempt to explain why Israelites had accepted the 7 day week of the sun worshippers.
The 7 day week originated in ancient Babylon (or perhaps earlier) in recognition of the 7 movable objects in the sky: The 5 movable stars (planets) plus the sun and the moon. Something as exceptional as stars that moved indicated to ancient minds that those stars must be gods -- so each star had to have a day dedicated to him. And the biggest object in the sky -- the sun -- had to have a day too. And as he was obviously the boss, his day had to be particularly holy. And to this day many of us regard Sunday as holy.
The Israelites didn't go down without a fight, however. They resisted the sun worshippers by saying in effect: "OK. If you celebrate the first day of the week as holy, we will celebrate the last day of the week as holy". And so they did and so they still do.
They were however stuck with the fact that everybody by then divided up the week into 7 days and they also knew perfectly well why. So they had to invent another story about how the 7 day week arose. Hence Genesis chapter 1. And the new story, of course, explained why the 7th day was particularly holy.
So it's all rather simple if you know your ancient history. What saddens me a little is that Christians have reverted to the old sun-worshippers day as their holy day.
Footnote: The account above is a basic outline but there are also some interesting details. Although Genesis chapter 1 is a late addition, it did not of course spring out of the blue. It would in fact seem to be the product of a very long debate.
The seven-day creation story is of course also mentioned in the ten Commandments of Exodus. And in that passage, the divine name IS used. So clearly, the story itself is much older than Genesis chapter 1. The Hebrews had to deal with sun-worshippers from the beginning so their retort to the sun-worshippers went back a long way too.