What does Genesis 17:21 mean?
Here, God concludes His revelations to the newly renamed Abraham for this encounter. Unlike earlier visits from God with their grand unconditional promises and few details, God's words to Abraham, now 99 years old, have been overwhelmingly specific.In addition to changing Abraham's name, God has commanded Abraham to walk before him and to be blameless. He has commanded Abraham to circumcise himself and all the males in his household. God has also changed Sarai's name to Sarah, revealing that she will bear a son and that son will be the one to whom God's covenant promises will pass. Ishmael, now 13, will be greatly blessed, but his will not be the covenant people of God.
Finally, in this verse, God reveals for the first time a timeline for his promises. By this time next year, a boy named Isaac will be born to Abraham and Sarah.
One can only imagine how much Abraham's head must have been spinning.