Where did Cain get his wife?

Short answer

The Bible does not name Cain's wife, but it says Adam and Eve had other sons and daughters, so she was most likely one of Cain's sisters or nieces. In the first generations of the human race there was no one else to marry.

The question looks like a trap. Genesis tells us that Adam and Eve had two sons, and one killed the other. Then Cain suddenly has a wife. Where did she come from? The text is short, but it gives enough to answer.

The text simply mentions her

After Cain leaves the presence of the Lord, the story moves on quickly.

And Cain knew his wife; and she conceived, and bore Enoch: and he built a city, and called the name of the city, after the name of his son, Enoch.
Gen 4:17 · King James Version (1769)

The New Geneva Study Bible points out that the Bible does not say who she was; it only states the fact. That is typical of Genesis. As Jamieson, Fausset and Brown note, this is a very brief history that skips many people and does not stop to explain every marriage.

Still, the book does not leave us entirely without a clue. Genesis tells us that Adam and Eve’s family was much bigger than the two sons we meet in chapter 4.

And the days of Adam after he had begotten Seth were eight hundred years: and he begot sons and daughters:
Gen 5:4 · King James Version (1769)

John MacArthur concludes from this verse that Cain’s wife was evidently one of the later daughters of Adam. Keil and Delitzsch read the text the same way: it takes for granted that she was a daughter of Adam and so a sister of Cain.

Time had passed

The story reads fast, but a lot of time lies between its lines. Albert Barnes notes that Abel already kept flocks, that Cain feared meeting people who would kill him (Genesis 4:14), and that Cain now had a wife and a growing family. All this suggests that a considerable time had passed since the fall.

Keil and Delitzsch add that centuries may have gone by before Cain built his city, and that the Hebrew word need mean no more than a fortified settlement. Adam and Eve had many children, and those children had children. The people Cain feared and the wife he married are both easy to understand in a family that was already large.

But was it not wrong to marry a sister?

Today marrying a close relative is rightly forbidden. The law of Moses later prohibited it (Leviticus 18:9). But that came long after Cain. Jamieson, Fausset and Brown say that such marriages were a necessity in the early days of the human race, and the law against them came much later. John MacArthur explains the later ban by the genetic damage that closely related parents can pass on. Keil and Delitzsch add that at the beginning the sons and daughters of Adam stood for the whole race, and the lines between families that later made such marriages sin had not yet formed.

Lopukhin’s commentary quotes John Chrysostom and Theodoret, who said the same thing: at the beginning the race had to multiply, so marriage between brothers and sisters was allowed. Lopukhin also thinks Cain probably married before he killed Abel, since after the murder few women would have joined him.

This explanation keeps to what the text says. The Bible gives no hint that there were other humans outside Adam’s family.

One family, one origin

This also matters for a larger truth. Eve is called the mother of all the living (Genesis 3:20). Paul tells the Athenians the same thing about all people.

And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation;
Acts 17:26 · King James Version (1769)

Albert Barnes explains that all the families of mankind come from one stock, however different they look or speak, and that no nation can claim to be better than another. MacArthur sees in the verse the equality of all people before God, since all come from Adam. The question about Cain’s wife therefore points to the unity of the human race: every one of us is a relative, and every one is equally in need of God’s grace.

The Bible does not dwell on the name of Cain’s wife because the chapter is about something else. Matthew Henry notes that Cain went out from the presence of the Lord and never found rest. The story shows where sin leads, and the wife in verse 17 is only a detail of how humanity grew.