Biblical explanation
Cain lived at a time when the human population consisted of descendants of the original family. The Bible does not provide a complete list of Adam's children. If the early human population descended from Adam and Eve, close-relative marriages would necessarily have occurred in the earliest generations. Later, the Mosaic law prohibited close-relative marriages. Leviticus 18:6-18. This law came many generations after Cain and therefore cannot simply be imposed retroactively on the earliest generations. Scripture does not tell us exactly who Cain's wife was. It gives enough information to understand that she was a descendant of Adam and Eve.
Key takeaway
Discipleship applies the gospel to the whole life: body, mind, relationships, work, finances, and worship. Healthy choices and faithful habits do not earn salvation; they are grateful stewardship of the life God has entrusted to us and practical ways to serve others.