To the end that the promise might be sure to all the seed. – Romans 4:16

Look at this, dear Friends, very carefully. Salvation was made to be of faith and not of works that the promise might be sure to all the seed, for first, it could not have been sure to us Gentiles by the Law, because in a certain sense we were not under the Law of Moses at all. Turn to the text and you find that it runs thus— “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.” That is to say, the Jew, receiving the seal of circumcision and coming under the ceremonial Law, eating its Passover and presenting its sacrifices, might possibly have been reached by a legal method. But we who are Gentiles would have been altogether shut out. As to the Covenant according to the flesh, we are aliens and have never come under its bonds or participated in its privileges and, therefore, Grace chooses to bless us by faith in order that the Gentile may partake of the blessing of the Covenant as well as the Jew.

But there is a still wider reason—it is of faith because the other method has failed, already, in every case. We have all broken the Law and so have put ourselves beyond the power of ever receiving blessing as a reward of merit. Failure at the outset has ruined our future prospects—and from now on—by the deeds of the Law shall no flesh be justified. If your standing depends upon your own works, you are not safe and can never be safe till you are out of this present life, for you might sin and that one offense against the conditions would destroy the Covenant! What remains, then, if we are to be saved at all, but that it should be of faith? This door, alone, is open! Let us bless God that no man can shut it!

How Is Salvation Received? by C. H. Spurgeon