How was he his only son at the time? Ishmael was still alive and well. I know the scripture, as well. I just can't understand how Isaac was considered the only one.
Ishmael was considered Abraham's seed. He was considered Hagar's son...
In a distant way, it would be vaguely like an adoption, Abraham was the biological father, yet God "adopted" him, and took care of him. God permitted Abraham to banish Ishmael.
Gen. 21:13 And also of the
son of the bondwoman will I make a nation, because he is thy seed.
Isaac was considered Abraham's son:
Genesis 22:2 says:
And he said, Take now thy son,
thine only son Isaac, whom thou lovest, and get thee into the land of Moriah; and offer him there for a burnt offering upon one of the mountains which I will tell thee of.
It was all in God's will. If God called Isaac, Abraham's only son, then that's who he was.
A somewhat similar situation happened with Jacob and Esau... Jacob obtained the birthright, while Esau was somewhat disowned. Both situations would be considered wrong by modern society's standards, But it was all God's will. In the end, Isaac was Abraham's only son at the time. Although he had other seed, he had one son at the time.