A.T. Robertson Commentary Romans 4:16

"For this cause [it is] of faith, that [it may be] according to grace; to the end that the promise may be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham, who is the father of us all" — Romans 4:16 (ASV)

Of faith (εκ πιστεως). As the source.

According to grace (κατα χαριν). As the pattern.

To the end that (εις το εινα). Purpose again as in 11.

Sure (βεβαιαν). Stable, fast, firm. Old adjective from βαινω, to walk.

Not to that only which is of the law (ου τω εκ του νομου μονον). Another instance where μονον (see verse 12) seems in the wrong place. Normally the order would be, ου μονον τω εκ του νομου, αλλα κα κτλ.