Alice C. Linsley
Abraham complained to the Lord that he didn't have a proper heir. This is the nature of his complaint, and one that the Lord God understood, seeing that Abraham was a ruler and the matter of heirs is especially important for those who rule. Why should having a proper heir be so important to Abraham were he simply an average man?
In his commentary on Genesis 15:2-6, Matthew Henry writes, "Though we must never complain of God, yet we have leave to complain to him; and to state all our grievances. It is ease to a burdened spirit, to open its case to a faithful and compassionate friend. Abram's complaint is, that he had no child; that he was never likely to have any; that the want of a son was so great a trouble to him, that it took away all his comfort." (From here.)
Matthew Henry is quite mistaken in some details. The Scriptures tell us that Abraham had eight other sons, many of these were born before Isaac. One of them was Eliezer, who would have served as an heir, though not the proper heir. Henry is correct that this was grievous to Abraham. Abraham regarded himself as cursed.
"O Lord God, what can you give me seeing that I shall die accursed, and the steward of my household is Dam-Mesek Eliezer?" (Genesis 15:2)
In providing a son, according to the divine promise, God overthrew the curse. Behold the pattern!
What constituted a proper heir for Abraham?
As Abraham was a ruler-priest, his proper heir was the firstborn son of his first wife, Sarah, who was also his half sister. In the Horite marriage and ascendancy pattern, the ruler's heir was the firstborn son of his half-sister wife. As Sarah was barren, the next in line was the firstborn son of a concubine. In this case, that was a Horite woman named Mesek. ("Dam-Mesek" means the one born of Mesek; the blood of Mezek.)
Analysis of the marriage and ascendancy structure of Abraham's Horite people reveals that the rulers had two wives. The first was a half-sister, as was Sarah to Abraham. The second wife was usually a patrilineal cousin, as was Keturah to Abraham. The wives maintained separate households in distant settlements on a north-south axis. Sarah resided in Hebron to the north, and Keturah resided in Beersheba to the south. These settlements marked the northern and southern boundaries of Abraham's territory.
Before Isaac was conceived, Abraham had taken a second wife, Keturah. Her firstborn son was Joktan (Yaqtan). He was never a potential heir for Abraham, however. The firstborn son of the cousin bride was named after the cousin bride's father as this son would serve as a sort of prime minister in the kingdom of his maternal grandfather.
Related reading: The Barren and Grieving Rejoice!; The Calling of Abraham; Abraham's Sons; Abraham's Firstborn Son; Abraham's Concubines; Cousin Brides and their Ruler Sons; Answers to More Questions