This is one of the most asked Sunday school questions of all time. The answer is straightforward — but worth taking seriously, because the question is honest.
What Genesis actually says
Genesis 5:4: "After he became the father of Seth, Adam lived 800 years and had other sons and daughters." The Bible names the sons that matter for the redemption narrative (Cain, Abel, Seth) and quietly notes there were many others — including daughters.
The text is not hiding anything. It is simply not interested in tracking everyone, because Genesis is the story of the line through which the promise of the Messiah is preserved.
The most natural reading
Cain married a sister or niece. In the early generations of humanity, this was inevitable. There was literally no one else.
"But isn't that incest?"
The Mosaic prohibitions against marrying close family members (Lev. 18) come much later — given to a large population where such marriages were no longer necessary, and where the genetic risks of close-kin marriage had accumulated.
The genetic point
Close-kin marriage today carries genetic risk because accumulated mutations make recessive defects more likely to show up in offspring. In the original humanity — whether one reads Genesis literally or as theological narrative — there were no accumulated mutations to worry about. There is no genetic reason the earliest generations of humans could not marry close kin.
This is consistent with what population geneticists know about bottlenecks: small founding populations work fine if the gene pool is clean.
The alternative readings
Some Christians (especially those who read Genesis in a more theological-narrative mode) suggest that Adam and Eve are presented as representative humans, with other populations existing alongside. This view tries to harmonize Genesis with archaeology suggesting widespread human populations in the relevant timeframes. It is not the traditional reading, but has been considered by C.S. Lewis, John Stott, and various Catholic theologians.
The traditional reading — Adam and Eve are the first humans; their descendants intermarried in early generations — remains the simplest answer to "where did Cain's wife come from."
The Orthodox lens
The Church does not dogmatize the precise mechanism of human origins. What the Church confesses: God created humanity; sin entered through human choice; death spread; the line to Christ was preserved through Seth. The theological point of Genesis 4–5 is what matters: God is faithful even when humans are not, and his covenant promise survives Cain's violence.
The Fathers (Chrysostom, Augustine) generally took the straightforward reading: Cain married a sister.
A pastoral word
If Cain's wife was the question that made you doubt Christianity, you can let it go. It has a quiet, traditional answer that has satisfied serious Christians for two thousand years.
The harder questions — Why does a good God allow evil? Did Jesus really rise from the dead? Why is there something rather than nothing? — are the ones worth your prayer. Those are where Christianity rises or falls. Cain's wife is not.