Fill ye up, then ... - This is a prediction of what they were about to do. He would have them act out their true spirit, and show what they were, and evince to all that they had the spirit of their fathers, Compare the notes at John 13:27. This was done be putting him to death, and persecuting the apostles.
The measure - The full amount, so as to make it complete. By your slaying me, fill up what is lacking of the iniquity of your fathers until the measure is full; until the national iniquity is complete; until as much has been committed as God can possibly bear, and then shall come upon you all this blood, and you shall be destroyed, Matthew 23:34-35.