Isaac was never called the “firstborn son of Abraham.” The fact is Abraham had two firstborn sons through two different women yet neither of them were ever called in Scripture “the firstborn son.” Ishmael was his firstborn son through Hagar an Egyptian maidservant and Isaac was his firstborn son through Sarah his wife. The story of Ishmael’s birth is given to us in Genesis chapter 16 and the story of Isaac’s birth is recorded in Genesis 21:1-7.

But this story began when Abram asked the Lord, “Lord GOD, what will You give me, seeing I go childless, and the heir of my house is Eliezer of Damascus?” (Genesis 15:2). The Lord responded by giving Abram an amazing promise, “This one shall not be your heir, but one who will come from your own body shall by your heir” (verse 4). He was even told he would have innumerable descendants (verse 5) and then we read, “And he believed in the LORD, and He counted it to him for righteousness” (verse 6). Yet as time went on his wife Sarai remained barren and she thought this promise could only be fulfilled through her maidservant Hagar, so she said to Abram, “See now, the LORD has restrained me from bearing children. Please, go into my maid; perhaps I shall obtain children by her” (Genesis 16:2). Abram took her advice and “Hagar conceived” (verse 4) and “bore Abram a son, and Abram named his son, whom Hagar bore, Ishmael” (verse 15). Abram was 86 years old at that time and the thought of Sarai bearing a child disappeared until God appeared to Abram when he was 99. He changed his name to Abraham and his wife’s name to Sarah and he gave Abraham this promise, “I will bless her and also give you a son by her” (Genesis 17:15-16). Both Abraham and Sarah laughed in unbelief because of their old age (verse 17 and Genesis 18:9-14) but God made good on his promise. In Genesis 21:1-3 we read, “And the LORD visited Sarah as He had said, and the LORD did for Sarah as He had spoken. For Sarah conceived and bore Abraham a son in his old age, at the set time of which God has spoken to him—whom Sarah bore to him, Isaac.” This is part of the story of these TWO FIRSTBORN SONS.

Now let’s read Genesis 22:1-2, “Now it came to pass after these things that God tested Abraham, and said to him, ‘Abraham!’ And he said, ‘Here I am.’ That He said, ‘Take now YOUR SON, YOUR ONLY SON ISAAC, whom you love, and go to the land of Moriah, and offer him there as a burnt offering on one of the mountains of which I shall tell you.” Abraham obeyed the Lord and as he was ready to offer up Isaac the Angel of the Lord called out to him and said, “Abraham, Abraham! …Do not lay your hand on the lad, or do anything to him; for now I know that you fear God, since you have not withheld YOUR SON, YOUR ONLY SON, from Me” (verses 11-12). Why did God call Isaac “your son, your ONLY son?” Ah, because Isaac was the true “SON of the promise” that God had in mind when He first promised Abram a son. This was confirmed hundreds of years later in the book of Hebrews where we read, “By faith Abraham, when he was tested, offered up Isaac, and he who had received the promises offered up his ONLY BEGOTTEN SON, who whom it was said, ‘In Isaac your seed shall be called’” (11:17-18 with Genesis 21:12).

In closing, Isaac was “the ONLY SON of Sarah and Abraham” and he was “the SON of the promise” given to Abram. It was a lack of faith on Abram and Sarai’s part that led to the birth of Ishmael. He was indeed a “son of Abram” but he was NOT the “son of promise” through which the nation of Israel would be born and eventually the promised Messiah of Israel and the Savior of the world. If there is a devout Muslim reading this, I know you believe Ishmael was the “son of promise” but we have just shown from holy Scripture that it was Isaac, not Ishmael, that God had in mind when He promised Abram a son, an heir who would come from his own body. I would encourage you to read Galatians 4:21-31 where it is confirmed again that Isaac was the “son of promise.”  (DO)  (672.3)