Cain knew his wife, and she conceived and bore Enoch... [Genesis 4:17]These words appear after Cain has been banished from his family's land, as punishment for the slaying of his brother Abel. It is most likely that Cain was married before he was banished, and hence the account here is simply when the couple began to bear children. What follows in chapter five up to chapter six and the flood details the sons of man (Cain's descendants) and the sons of God (Seth's descendants).
Cain's wife had not been previously mentioned nor named - indeed, neither was anyone else other than Adam and Eve, Cain and Abel. This, however, does not presume they did not exist, as the author of Genesis had a point in focusing on a select group of people. John Calvin writes that "many persons, as well males as females, are omitted in this narrative; it being the design of Moses only to follow one line of his progeny, until he should come to Lamech." Genesis 5:4 mentions that, besides Cain, Abel, and Seth, Adam and Eve begat many other sons and daughters. Therefore, although other human beings were not named specifically, this first generation of man was much larger than simply four people.
The situations around our verse presents what is to many a large stumbling block: just where did Cain's wife come from? If there were no other women outside the immediate family unit, does that mean he married his sister? These are difficult questions for certain, and from them arise two possible explanations for Genesis 4:17:
1) These women existed outside of the Adam/Eve family unit. This can go one of two ways: a) these women were made by God in the same manner Adam and Eve were; b) these women existed in populations outside Adam and Eve. However, if we permit either of these, we must then ask what becomes of the federal headship of Adam, and how his sinful nature carried down into the rest of mankind. One would have to explain the words of the apostle Paul when he wrote: "Therefore, just as sin came into the world through one man, and death through sin..." (Rom 5:12). This would also affect the previously mentioned dual lineage seen in chapter five.
2) These women were sisters of Cain and Abel. This will, to many, sound strange, given the ban on incest in the Mosaic Law (Deu 27:22). Many commentators say the reason it was permitted here was necessity, as well as the fact humankind had not fallen into such a state that the health ramifications were a problem. As Albert Barnes writes in his commentaries:
The wife of Cain was of necessity his sister, though this was forbidden in after times, for wise and holy reasons, when the necessity no longer existed.David Guzik likewise writes in his commentary:
We don’t know where did Cain got his wife. Genesis 5:4 says Adam had several sons and daughters. Cain obviously married his sister. Though marrying a sister was against the law of God according to Leviticus 18:9, 18:11, 20:17, and Deuteronomy 27:22 (which even prohibits the marrying of a half-sister), this was long before God spoke that law to Moses and the world. Here, necessity demanded that Adam’s sons marry his daughters. And at this point, the “gene pool” of humanity was pure enough to allow close marriage without harm of inbreeding. But as a stream can get more polluted the further it gets from the source, there came a time when God decreed there no longer be marriage between close relatives because of the danger of inbreeding.And the Keil and Delitzsch Commentary on the Old Testament writes:
The marriage of brothers and sisters was inevitable in the case of the children of the first men, if the human race was actually to descend from a single pair, and may therefore be justified in the face of the Mosaic prohibition of such marriages, on the ground that the sons and daughters of Adam represented not merely the family but the genus, and that it was not till after the rise of several families that the bands of fraternal and conjugal love became distinct from one another, and assumed fixed and mutually exclusive forms, the violation of which is sin. (Comp. Leviticus 18.)John MacArthur writes in his commentary:
Cain's wife obviously was one of Adam's later daughters (5:4). By Moses' time, this kind of close marriage was forbidden (Lev 18:7-17), because of genetic decay.Hank Hanegraaff, in an article related to this very subject, writes:
Furthermore, because genetic imperfections accumulated gradually over time, there was no prohibition against incest in the earliest stages of human civilization. The Levitical law against incestuous relationships was given by God hundreds of years after Cain at the time of Moses. Thus familial relationships were preserved and birth defects were prevented (Leviticus 18:6, 9). [source]We might conclude from this, then, that at this time it was not only a necessity, but that the human condition had not fallen into such a state that the medical effects of incestuous relationships were worthy of concern. We can definitely see a steady decline in the state of man's health up to the time of Abraham, if ages are to mean anything: Adam lived to be 930-years old (Gen 5:5), while Abraham lived to be 175-years old (Gen 25:7).
John Gill, in his commentaries, presents what may seem to some a more reasonable possibility. Namely, that this woman Cain married could have "descended from Adam by another of his sons, since this was about the one hundred and thirtieth year of the creation." This still does not, however, answer how the other, unnamed son reproduced, and we would have to assume it was through a sister. Therefore, we return to the dilemma we started with.
John Gill likewise mentions a tradition among the Jews and others, that Cain and Abel each had twin sisters when they were born, and that one married the other.
At first indeed Cain could marry no other than his sister; but whether he married Abel's twin sister, or his own twin sister, is disputed; the Jews say, that Cain's twin sister was not a beautiful woman, and therefore he said, I will kill my brother and take his wife: on the other hand, the Arabic writers say, that Adam would have had Cain married Abel's twin sister, whom they call Awin; and Abel have married Cain's twin sister, whom they call Azron; but Cain would not, because his own sister was the handsomest; and this they take to be the occasion of the quarrel, which issued in the murder of Abel.This would still present something of a problem, as marrying a sibling's twin sister is still technically marrying a sibling - hence, once again, we are back to the original conundrum.
After reviewing this situation and the various solutions, the most likely answer is that Cain's wife was an unnamed daughter of Adam and Eve, being Cain's sister. At the time, this was permissible, both for necessity's sake, as well as for the simple reason man had not fallen into such a miserable condition as they are today.
Many will here ask why God would permit it in one moment, yet ban it later on. God, however, permits and forbids actions or deeds according to His purpose and according to the timeliness of the ban or permit. For example, ravens were declared unclean diet for the Jews under the Mosaic covenant (Deu 14:14), and yet ravens were earlier saved from the flood due to God's mercy on His created animals (Gen 8:7). In fact, there were many unclean foods the ancient Jews were banned from eating under the Mosaic covenant, and yet they are permitted under the covenant with Christ (Mark 7:19). Another example, the looking upon a bronze snake was permitted by God for a temporary time (Num 21:9), but banned later on when it became idolatrous (2 Ki 18:4). These are not contradictions any more than a law enacted by a civil government that limits or bans something which was at one point permitted but has now become an issue or a problem. A city, for example, may keep a public park open 24/7, but because of crimes at night enact an ordinance that bans activity in the park after a certain hour. That, however, is not a "contradiction," and those who argue as such are ignoring the circumstances around the change.