Be heirs (κληρονομο). No predicate in the Greek (εισιν). See on Galatians 4:1. If legalists are heirs of the Messianic promise to Abraham (condition of first class, assumed as true for argument's sake), the faith is emptied of all meaning (κεκενωτα, perfect passive indicative of κενοω) and the promise to Abraham is made permanently idle (κατηργητα).