After Cain murdered his brother, Abel, the Lord disciplined him by making him “a restless wanderer on the earth” (Gen. 4:12). Nevertheless, God showed Cain grace in the midst of judgment by placing “a mark on Cain so that no one who found him would kill him” (Gen. 4:15). Cain then settled “in the land of Nod, east of Eden,” married, and had a son (Gen. 4:16–17).
If Cain was one of only three sons born to Adam and Eve, where did Cain find this wife? And a related question is who else existed who would potentially want to kill him?
The answer lies in the nature in biblical storytelling, called narrative. One characteristic of the narrative of Scripture is that it is intentionally selective in the information it transmits. Biblical narrative tells only what needs to be known for the story. In the account of Cain and Abel, it was unnecessary to include a description of any other siblings. That does not mean, however, that Adam and Eve did not have other children, both male and female. Indeed, God’s command, “Be fruitful and increase in number; fill the earth and subdue it,” means it is likely that Adam and Eve had many more children than just the three whose names we are given (Gen. 1:28).
This explains both questions. First, Cain likely feared that his brothers (and possibly also nephews) would want to kill him to avenge the murder of their brother Abel. Thus, God marked Cain to protect him. Second, Cain found a female descendant of Adam and Eve to marry. We don’t know how much time had elapsed since the eviction from the Garden of Eden or how many descendants of Adam and Eve now filled the world, but it seems like a clear conclusion from this text that the population was sufficient to provide Cain both an option to marry and a reason to fear.