The most reasonable answer within Christianity is that Cain married some other descendant of Adam and Eve's. If God created Adam and Eve to be the first truly human beings, then it seems most natural to conclude that God had meant for close relatives to pair off and form father-mother units in the early generations of humanity. Given the fact that God created Adam and Eve, and they were exhorted and blessed with the words "Be fruitful and multiply", it is reasonable that the descendants of Adam and Eve could find mates.

A common objection to this answer is that up to Gen 4:17 no daughters have been mentioned, so where could Cain get his wife? This objection assumes that the transition from 4:16 to 4:17 is one of an immediate chronological nature.

To see that this assumption is not reasonable, observe that Adam's entire life has been compressed into Genesis 2:8-5:3. We have a very densely compressed narrative here in the early part of Genesis. We have no literary guarantee that the events of 2:21 happened immediately after Adam was done naming the animal kingdom. Nor do we have any right to claim that 4:1 immediately follows 3:24.

Given these facts, that the early chapters of Genesis are merely reporting key events in the early history of humankind, we have no right to take our biographical standards of a smooth lineal flow of time and apply them to a Semitic document some 2500-3500 years old.

Second, it assumes that what happens in later verses must necessarily occur after what happens in earlier verses. Adam is reported to have lived 930 years -- plenty of time for fathering many children. In fact, Gen 5:4 reports that he had sons and daughters. This passage, being located in a summary account of the line from Adam to Noah, cannot be pinned down to a specific time posterior to Cain's exile and hence one cannot be dogmatic concerning the thesis that Cain could not have found a wife.

The passage 5:4 speaks of a process: Adam had sons and daughters. This is not the reporting of a punctiliar event, but instead is the reporting of a series of events. Thus, it is quite possible and reasonable to conclude that Adam had other children throughout Cain's duration.

What is more, the children of Adam and Eve would have their own children, and those children would propagate, and so on. Therefore, Cain would not at all be restricted to selecting a sister of his for a mate. He may have chosen a niece, a grand niece, a grand-grandniece, or so on. Note that 4:17 does not specify precisely when Cain found a mate. It merely assumes that Cain had found a mate, and does not provide an exact date relative to the other mentioned events.

An objection to the classic solution is that this relationship would be incestuous. If God created one man and one woman and exhorted them to multiply, their children would intermingle and propagate humanity. Apparently, such actions were not sinful then.

It is true that in Leviticus 18 there is a set of laws against certain types of sexual relationships. Relationships between brother and sister were permitted in the very early days of humanity but were not permitted in this code, which was given during the Exodus, long after Cain had been around. God permitted a certain action early on, and later no longer permitted it.

Some may argue that this implies morality evolves, equating it to 'situational ethics.' However, it is important to distinguish between humans deciding what is ethical (situational ethics) and God establishing moral boundaries at different periods. It is a category fallacy to equate God's unfolding redemptive historical commands with 'situational ethics.' This confuses a scenario where human beings subjectively decide what is ethical with a situation where God, the objective moral lawgiver, decides what is permitted or forbidden in a given epoch.

The fact that God allows a certain practice at one time but later forbids it in no way increases the autonomy of humanity with respect to a moral law given from outside humanity. If the moral law originates exterior to man, then humans cannot change the law, even if the divine decrees change across redemptive history.

While Scripture does not give a clue as to exactly how he managed to obtain his wife, a lack of knowledge concerning this historical detail does not invalidate the coherence of the text. Furthermore, reasonable deductions are not mere guesses, as they are based on explicit propositions within the text.

Problem: There were no women for Cain to marry. There was only Adam, Eve (4:1), and his dead brother Abel (4:8). Yet the Bible says Cain married and had children.

Solution: Cain married his sister (or possibly a niece). The Bible says Adam “begot sons and daughters’’ (Gen. 5:4). In fact, since Adam lived 930 years (Gen. 5:5), he had plenty of time for plenty of children! Cain could have married one of his many sisters, or even a niece, if he married after his brothers or sisters had grown daughters. In that case, of course, one of his brothers would have married a sister.