In Genesis 22-2, God refers to Isaac as Abraham's "only son". I have heard explanations asserting that its because Ishmael was a born of a slave woman, but it still doesn't mean that God would altogether disregard Ishmael as a son. Is it a rule in the Bible that sons born to slave women are illegitimate, and not regarded as true sons ?
Paul writes, in Galatians 4:22 [AV] that ‘Abraham had two sons, the one by a bondmaid, the other by a freewoman . . which things are an allegory.' These things were set forth, in reality, in antiquity, by God’s providence, that there might exist an allegory in which truth was demonstrated. And Paul opens up the doctrinal truth of the historic demonstration in the epistle to the churches of Galatia.
In Genesis 22:1 in the AV, the second time the word ‘son’ is used it is in italics, indicating that the Hebrew word ben is not there in the original. Young’s literal and Green’s Hebrew Interlinear indicate the same :
Take now thy son, thine only Isaac . . . . [AV] [minus italic word]
Take, I pray thee, thy son, thine only one . . . [Young]
Take now your son, your one only . . . .[Green]
Not only is it true that Abraham had two sons, but it is also true that he had a 'one only' - because, by this time, God had instructed Abraham to do as Sarah desired, Genesis 21:10 & 12, and to cast out the troublesome Ishmael together with her mother. Leaving Abraham with 'one only', in the sense of a familial relationship and a proper heir.
Thus the one-onliness is a fact, but it is separated from the word 'son' in the text. This resulting wording conveys three matters in the New Testament. The first is the fact that Abraham’s willingness to sacrifice Isaac stands out as a demonstration of God’s willingness to sacrifice his ‘one only’, his only begotten Son.
The second is that Isaac, the one-only of Abraham was willing to be sacrificed. It became clear, in the latter stage of the proceedings, what Abraham’s intentions were and the thirteen year old boy, quite capable - by that age - of looking after himself, appears to have complied with them. Thus, if so, this demonstrates the willingness of the Son of God, God’s one-only, to be the Testator of the Everlasting Testament.
The third matter, which can be thus accommodated within Abraham’s life and family circumstances, along with the first two matters, is the allegorical nature of that which Paul draws attention to - the matter of two seeds, with two destinies, and two mountains (Sinai and Sion), and two cities (Jerusalem below which is in bondage with her children and Jerusalem above which is free) and two covenants (the old legal covenant with Israel which was but for a time and the Everlasting Testament which will never cease).
Hagar and Sarah, Sinai and Sion, Jerusalem above and below.
What a wealth of allegory and instruction is gathered around Abraham ! What depths are discovered in the way in which God called him, preserved him, guided him and - above all - set forth in himself and his family such a wonderful display of allegorical instruction for the generations who have followed.
Thank you for your question. It has been a delight to consider these things this morning.
As Isaac was the only one son with him on that time. This is actually the second test. The first test was to leave Ishmael and Abraham was loyal to God. Now Abraham is left with only one son. The second test (Genesis 22:2) is to sacrifice him as well. Abraham was loyal to the God this time also.
Note: Ishmael is also blessed (Genesis 21:18). It means God loves everyone.