All total, Abraham had eight sons.
Abraham’s first son was Ishmael through Hagar, his wife’s Egyptian maid (Genesis 16:1–4).
Abraham’s second son was Isaac through Sarah, his wife (Genesis 21:1–3). Isaac was the son God had promised Abraham (Genesis 15:4–5).
After Sarah died, Abraham had six sons through Keturah, another concubine: Zimran, Jokshan, Medan, Midian, Ishbak, and Shuah (Genesis 25:1, 6). Keturah’s sons became the fathers of Arabian tribes living east of Israel.
Some people claim that the Bible makes an error in regards to the number of Abraham’s sons. In Genesis 22:2, God speaks to Abraham after the birth of Ishmael, referring to Isaac as “your son, your only son, whom you love.” Then Hebrews 11:17 identifies Isaac as Abraham’s “one and only son.” And Galatians 4:22 mentions only Isaac and Ishmael: “It is written that Abraham had two sons.” How could Abraham be said to have an “only son” and “two sons,” when in reality he had eight sons?
There is no true contradiction in the above passages. Isaac was the only son who was promised to Abraham and through whom Abraham would become the father of many nations (Genesis 12:1–3; 17:1–8; 21:12). Also, Isaac was the only son of Sarah and Abraham—Sarah being specifically mentioned in the prophecies of Genesis 17:16–21 and 18:10. In addition, Isaac is the only son born in an official marriage: Hagar and Keturah were both concubines. While God blessed the concubines’ sons for Abraham’s sake, those sons had no part in the inheritance. Isaac was the one and only rightful heir to the promise (Genesis 15:4–5; 25:5).
Genesis 22:2 and Hebrews 11:17 both refer to Isaac as Abraham’s “only son” because those passages concern God’s promise and covenant. Since Abraham’s other seven sons are not part of the covenant, they are irrelevant to the issue and not mentioned as sons. Abraham had other sons, but only one son of promise.
The main theme in Galatians is justification by faith, apart from the Law. Galatians 4:22 mentions only two sons, Isaac and Ishmael, in an allegory to highlight the contrast between the old covenant of law and the new covenant of grace. The former leads to bondage while the latter to freedom and life. Paul’s reasoning is as follows: Ishmael was the son of Hagar, a slave, and thus symbolizes bondage and slavery to the Law. Ishmael was the product of a human effort to bring about God’s blessing; Ishmael equals the works of the Law. Isaac was born to the free woman, Sarah, and thus symbolizes freedom and life. Isaac was born in God’s time, according to God’s promise, without the scheming or interference of man; Isaac equals the gift of grace. This passage in Galatians 4 is meant to teach a spiritual lesson (verse 24), not to give a detailed account of Abraham’s life and how many actual sons he had. Mentioning the other six sons would not have served any meaningful purpose in Paul’s allegory.
Spiritually speaking, Abraham has many, many sons. The Bible points to the faith of Abraham (Genesis 15:6) and states that “those who have faith are children of Abraham” (Galatians 3:7; cf. verse 9). Those who exercise the same faith that Abraham had are showing themselves to be like him, spiritually, and so can be rightly called his “children.” All who trust in Christ, as Zacchaeus did, become true sons of Abraham (Luke 19:9). “The promise comes by faith, so that it may be by grace and may be guaranteed to all Abraham’s offspring . . . to those who have the faith of Abraham. He is the father of us all” (Romans 4:16).