It seems that this is one of those subjects that have perplexed individuals for ages; Christians because there is no one definitive verse that clearly states who she was and agnostics who view this supposed omission as an alibi to “wrest the scriptures to their own destruction” (2 Peter 3:16). However, if we compare scripture with scripture we can identify Cain’s wife with some simple biblical deduction and a little sound reasoning.
Some have suggested that there could have been other humans not begotten from Adam and Eve’s union around at this time but this simply cannot be true for Genesis 3:20 states that Eve was the, “mother of all living.” According to the Biblical record, we also know that they were the only created human beings (Genesis 2:7, 22). Therefore, any humans that existed at that time had to be born from their union. This then leaves us with only one plausible explanation as to the identity of Cain’s bride; it must have been his sister.
The Bible doesn’t mention them having any daughters until Genesis 5:4 but obviously Eve was bearing them at or near the same time as Cain and Abel for in Genesis 4:17 Cain is said to already have a wife. In addition, common sense dictates that these daughters had to be born near or around the same time as the sons or else there would have been no way for the human species to procreate. With the Lord’s command for Adam and Eve, “to be fruitful and multiply,” (Genesis 1:28) and with their long life expectancy it was possible (if not probable) that Eve was able to bear more children (quintuplets, sextuplets, etc.) and at a faster rate then is possible for women presently. This would make the most sense in light of Genesis 3:16 where the Lord told Eve, “I will greatly multiply…thy conception.” Note that the first time Cain is said to know his wife that there is already enough of a population existing for him to establish and name a city after his firstborn son Enoch (Genesis 4:17).
As to the problem with Cain marrying his sister this can be answered once again by searching the scriptures instead of relying upon our preconceived prejudices. At this time in the Bible it was permissible for a man to marry a sister or first cousin for in Genesis 20:12 Abraham is said to have married his sister Sarah. It was not until the Law was given to Moses (see Leviticus 18:6-17, 20:17) that the marrying of blood relatives was forbidden. An interesting side note to this study is that a New Testament wife (1 Corinthians 9:15) and the bride of Jesus Christ (Song of Solomon 2:2, 4:9) are both referred to as a sister.