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.
Whose children ye are ...  Paul extensively developed the thought of Christians being the children of Abraham, a principle given by Christ himself (John 8:39ff); and this is a further extension of the same truth. Being sons of Abraham, as all Christians are  (Galatians 3:29), they are also children of Sarah, Abraham's wife.