as Sarah obeyed Abraham, calling him lord, whose children you now are, if you do well, and are not put in fear by any terror.
If ye do well ...  This qualifier stands over against all Christian privilege.  The thing that disqualified the Jews of Jesus' day as true sons of Abraham was disobedience; and the Christian must accept the application of the same principle to the members of the new Israel.  If they do not do well, they shall become, like the disobedient Jews of Jesus' day, "sons of the devil" (John 8:44).