Alice C. Linsley
Abraham said, O my Lord, what would you give me seeing that I am going to die accursed [Hebrew ariri], and the one to inherit my household is Dam-Mesek." Genesis 15:2
A reader has raised this probing question in reference to this essay on Eliezar, son of Masek:
Granted that biblical events are not always listed in strict chronological order, but one aspect of the original question you refer to does not seem to be addressed at all. The writer stated that Ishmael had already been born. But this concern Abraham expressed about a possible future heir occurs before the birth of Ishmael, who does not come on the scene until later in the narrative.
Isaac was born after the conversation Abraham had with God about Eliezer being the heir to Abraham's throne in Genesis 15. In the passage cited above God reassures Abraham that he will have a son by Sarah. In the marriage and ascendancy structure of Abraham's people, the firstborn son of the half-sister wife ascended to the throne of his biological father. Sarah was Abraham's half-sister, They had the same father (Terah) but different mothers.
Abraham had a firstborn son already by his cousin wife Keturah. However the firstborn son of the cousin wife rules in the territory of his maternal grandfather, after whom he was named.
Ishmael had already been born, but the firstborn son of a concubine would not be an heir to the throne of his father if their were a firstborn son by a half-sister wife. Ishmael would have been Abraham's heir by adoption according to Horite/Hurrian law. According to this practice, the concubine gave birth on the lap of the half-sister wife, as was the case with Hagar's delivery of Ishmael, whereby Sarah built a family through Hagar (Gen. 16:2).
By this time Abraham was well-established as a "mighty prince" (Gen. 23:6) among the Canaanites. That is inferred from Abraham's defeat of the kings who had attacked his Horite people (Gen. 14:6) and the status of being served by Melchizedek, the ruler-priest of Salem (Gen. 14:18), who with wine and bread, came to make atonement for Abraham's blood guilt and to give thanks for the victory.
Remember that Abraham had two wives, as was the pattern for all Horite rulers. His father Terah had two wives. Sarah was Terah's daughter by one wife and Abraham was Terah's son by the other wife. This pattern of two wives meant that there were always two firstborn sons; one by the half-sister wife and the other by the cousin/niece wife. The firstborn son of the cousin/niece wife ascended to the throne of his maternal grandfather, after whom he was titled/named. So Esau the Younger ascended to the throne of Esau the Elder (Gen. 36).
The firstborn of the half-sister wife ascended to the throne of his biological father, so Isaac was Abraham's heir. However, he was not Abraham's firstborn. Neither was Ishmael. Ishmael was conceived late in Abraham's life, after Abraham had married Keturah. Keturah's firstborn son was Joktan (Yaqtan), the head of the Joktanite Tribes of Arabia. As Keturah was Abraham's cousin wife, Joktan ascended to the throne of his maternal grandfather. As Sarah was barren, Abraham was still desperate for an heir. This is when he prayed about having an heir and received the promise (Gen. 15:4) that a son would come from his own "loins" (meaning blood descent from him and his half-sister).
Eliezar, as the firstborn of Masek, one of Abraham's concubines, was Abraham's only natural heir. Clearly before Isaac arrived, Eliezar was considered Abraham's rightful heir according to the Horite marriage and ascendency pattern.
The name Eliezar/Eleazar appears twice in the Horite ancestry of Jesus Christ.
Related reading: Abraham's Complaint; Who Were the Horites?; The Marriage and Ascendency Pattern of Abraham's People; Abraham's Cousin Wife; Abraham's Concubines; Answers to More Questions