Was Issac Abraham’s only son?
That might be a harder question to answer than it first appears. It is one of those questions where it is best not to answer “yes” or “no” because both can be greatly misleading.
Let’s start at the beginning. We first meet Abraham (Abram at that point) in Genesis 12. As part of His call, the LORD blesses him and promises to make him a great nation (12:2) and that his offspring will inherit the land (12:7). In chapter 13, the LORD reiterates the promise of offspring and states that they will be as numerous “as the dust of the earth” (13:16). Two chapters later, the LORD clarifies the blessing by promising a son as heir (15:4).
The LORD demonstrates His faithfulness and generosity to Abraham and Sarah in Genesis 21 as Isaac is born. The long wait is finally over, and Sarah’s once hesitant and skeptical laughter now overflows with joy. A son is born, a son who is later called Abraham’s “only son” (Genesis 22:6, 16).
But what about Ishmael?
In Genesis 16, long before the angel’s appearance and the birth of Isaac, Sarah had attempted to fast forward God’s promise by giving her maidservant to Abraham. As a result, “Hagar bore Abram a son” (16:15).
How then is it that Isaac could be called Abraham’s “only son ” if he had another son? The Bible is surely not incorrect or contradictory. Instead, we find a hard yet beautiful truth if we dig deeply enough. As Paul writes in Romans 9, “not all are children of Abraham because they are his offspring.”
Inherent to the concept of sonship is inheritance. To be a son in the biblical sense is to be an heir. Ishmael, though loved by Abraham and an offspring from his body, was not the promised heir. Isaac alone was the heir. He was the “only son.” Even Abraham’s other children (Genesis 25:1-2) were not called his sons in the sense that Isaac was. God promised him only one son.
Dig a little deeper still and find that even Isaac was not the ultimate fulfillment of the promise. When we read the New Testament, light pours in, and shadows recede so that the substance can shine forth brilliantly (Colossians 2:17; Hebrews 8:5, 10:1). And Christ is the true and greater heir of God’s promises (Galatians 3:16). He was the “only Son” Who died upon a mountain at the hands of His Father. He was the “only Son” through Whom grace, mercy and blessing would flow to the nations.
And oh what glory and mystery to further apprehend that you and I are called offspring of Abraham (Galatians 3:29), not through physical descent but through union to Christ, the fulfillment of all God’s promises (2 Corinthians 1:20). As Abraham’s sons and daughters, we are therefore sons and daughters of the good and generous Father through faith in the One true and faithful Son.