John Calvin Commentary Genesis 17:15

"And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be." — Genesis 17:15 (ASV)

As for Sarai thy wife, God now promises to Abraham a legitimate seed by Sarai. She had been (as I have said) too hasty, when she substituted, without any command from God, her maidservant in her own place. Abraham also had been too compliant in following his wife, who foolishly and rashly wished to anticipate God's design; nevertheless, their shared fault did not prevent God from making it known to them that he was about to give them that seed, from the expectation of which, they had, in a way, cut themselves off.

Therefore, God's undeserved kindness shines all the more clearly because, although people hinder its course with their own obstacles, it still comes to them.

Moreover, God changes Sarai's name to extend her prominence far and wide, which in her former name had been more restricted. For the letter י (yod) has the force of the possessive pronoun among the Hebrews: this now being removed, God intends for Sarah to be celebrated everywhere, and without exception, as a sovereign and princess.

And this is expressed in the context when God promises that he will give her a son, from whom nations and kings would eventually be born.

And although at first sight this blessing appears most abundant, it is still far richer than it seems in the words used here, as we will see shortly.