A.T. Robertson Commentary Romans 4:14

"For if they that are of the law are heirs, faith is made void, and the promise is made of none effect:" — Romans 4:14 (ASV)

Be heirs (κληρονομο). No predicate in the Greek (εισιν). See on Ga 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).