Romans 4:16

Therefore it is of faith that it might be by grace
Meaning either the promise of being heir of the world, or the inheritance itself, or adoption which gives heirship, or remission of Sin, or the blessing of justification, either and all of these are of faith; not as the cause or condition of them, but as the means of God's fixing and appointing to be the recipient of all and each of them: which is done, "that it might be by grace"; appear to be of the free grace and favour of God, as each of these blessings are: forasmuch as every blessing is received by faith, it is manifest it must be by grace; since faith itself is a gift of God's grace, and lies purely in receiving favours at the hand of God, to whom it gives all the glory of them: and this is done with a further view,

to the end the promise might be sure to all the seed.
The promise of the above blessings, particularly of the inheritance which is made in the covenant of grace, ordered in all things and sure, and which could not be disannulled by the law that came after it; this being by faith and of grace, and not of works, nor at all depending upon them, becomes sure to all believers, to all Abraham's spiritual seed:

not to that only which is of the law;
to the Jews, who are said to be of the law, in distinction to the Gentiles who were without it; and designs such of them as were believers in Christ, and to whom the Gospel was the power of God unto salvation; to these the promise was, and was sure, and not to them only:

but to that also which is of the faith of Abraham;
to the Gentiles, who though they are not by natural descent from Abraham, yet are of the same faith with him, and so are his seed in a spiritual sense:

who is the father of us all;
whether Jews or Gentiles, who are Christ's, and so Abraham's spiritual seed, and heirs of eternal life, according to the free promise of grace.