I have trouble believing that Adam and Eve were the only people God created. First, sure, but not only. Genesis 4 tells us that Eve had Cain, and then Able. Genesis is very methodically about the whole begetting thing, so why would other children be left out?
And once Cain kills able, it says "“Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me. 15: And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.”
Who are these people who’ll find and kill him? He doesn’t seem to have any more siblings until “And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. 17: And Cain knew his wife; and she conceived, and bare Enoch — 25 : And Adam knew his wife again; and she bare a son, and called his name Seth: For God, said she, hath appointed me another seed instead of Abel, whom Cain slew.”
If we are to assume that those people in Nod are Adam and Eve’s unmentioned kids (and why would they be unnamed? Enoch and Seth don’t seem to do much compared to their older brothers, and they’re named) that’d mean that Cain and Able were tremendous slackers compared to their unnamed younger siblings that were off building a town.
And why was Seth a notable replacement for Able if they’d had a lot of kids we don’t know by name? (actually, I can’t figure out why Enoch isn’t “the replacement” since he was born next.)
I don’t know, I have more difficulty believing that the younger siblings, at least some of them sisters, were off building a city while Cain and Able farmed for Mom and Dad than I do believing that God created some people in Nod too, but we don’t learn about them because they didn’t do anything considered too imporatant by the scribes who wrote out the bible.