I find the idea behind circumcision in the Old Testament fascinating. God tells Abraham to be fruitful and multiply, but then as a sign of this covenant, God symbolically “cuts off” (circumcizes) the means by which Abraham produces the seeds to be “fruitful” (Genesis 17:10-11).
When the son of the promise (Isaac) is born, he’s the only person who is born without the husband “knowing” his wife (until Mary and Christ, that is). It just says that “the LORD did to Sarah as he had promised.” (Genesis 21:1-2).
Of course, this son of the promise (Isaac) is the only Old Testament patriarch to have been born in the promised land, stay in the promised land, and die in the promised land (also a part of the covenant; Genesis 15:18-20).
In this way, Isaac becomes the patriarch par excellence, and the sign pointing to Christ (remember, Isaac is the Son whom Abraham is asked to sacrifice).
But, it seems to also show that the covenant is God doing what he said he would (he makes us fruitful, through his word), rather than us making ourselves out to be a success.