Beloved are the converts, and the Mekhilta offers a stunning proof: God delayed Abraham's circumcision until the age of ninety-nine specifically to keep the door open for future converts.
The reasoning is practical. If Abraham had been circumcised at twenty or thirty, a precedent would have been set. Future converts might have been told: you must convert before age thirty, just as Abraham was circumcised before thirty. Anyone older would be turned away.
By waiting until Abraham was ninety-nine, God ensured that no age barrier could ever be imposed on conversion. If the father of the Jewish people entered the covenant at ninety-nine, no one could argue that a sixty-year-old or seventy-year-old was too old to become a Jew.
This teaching reveals something extraordinary about divine planning. God's decision about when to command Abraham's circumcision was not about Abraham alone. It was about every person who would ever seek to join the Jewish people in the centuries and millennia to come. God looked forward through all of history and saw future converts who might be excluded by an age requirement. He removed that barrier before it could form, by making the founding precedent as inclusive as possible.
The Mekhilta treats God's love for converts as so foundational that it shaped the very timeline of the patriarch's life. Abraham waited ninety-nine years so that no convert would ever have to wait too long.