“Come now.” For this translation compare Anchor Bible Commentary (Joseph Fitzmyer).^a NASB, HCSB, and KJV translate the oun (#3767 οὖν) as “therefore.” But “therefore” normally indicates the practical application of that which came before, which makes no sense in this context; rather, this is a continuation of narrative, a logical connection, not properly the practical application. “Come now” captures this sense well.

“these stones.” The Jews claimed that salvation was in large part due to their being descendants of Abraham, and John was repudiating that belief (see commentary on Matt. 3:9).