Genesis 28:14

14.And thy seed shall be as the dust of the earth. The sum of the whole is this, Whatever the Lord had promised to Abraham, Jacob transmitted to his sons. Meanwhile it behoved the holy man, in reliance on this divine testimony, to hope against hope; for though the promise was vast and magnificent, yet, wherever Jacob turned himself, no ray of good hope shone upon him. He saw himself a solitary man; no condition better than that of exile presented itself; his return was uncertain and full of danger; but it was profitable for him to be thus left destitute of all means of help, that he might learn to depend on the word of God alone. Thus, at the present time, if God freely promises to give us all things, and yet seems to approach us empty-handed, it is still proper that we should pay such honor and reverence to his word, that we may be enriched and filled with faith. At length, indeed, after the death of Jacob, the event declared how efficacious had been this promise: by which example we are taught that the Lord by no means disappoints his people, even when he defers the granting of those good things which he has promised, till after their death.

And in thee, and in thy seed, shall all the families of the earth be blessed (58) This clause has the greater weight, because in Jacob and in his seed the blessing is to be restored from which the whole human race had been cut off in their first parent. But what this expression means, I have explained above; namely, that Jacob will not only be an exemplar, or formula of blessing, but its fountain, cause, or foundation; for though a certain exquisite degree of happiness is often signified by an expression of this kind; yet, in many passages of Scripture, it means the same as to desire from any one his blessing, and to acknowledge it as his gift. Thus men are said to bless themselves in God, when they acknowledge him as the author of all good. So here God promises that in Jacob and his seed all nations shall bless themselves, because no happiness will ever be found except what proceeds from this source. That, however, which is peculiar to Christ, is without impropriety transferred to Jacob, in whose loins Christ then was. Therefore, inasmuch as Jacob, at that time, represented the person of Christ, it is said that all nations are to be blessed in him; but, seeing that the manifestation of a benefit so great depended on another, the expression in thy seed is immediately added in the way of explanation. That the word seed is a collective noun, forms no objection to this interpretation, (as I have elsewhere said,) for since all unbelievers deprive themselves of honor and of grace, and are thus accounted strangers; it is necessary to refer to the Head, in order that the unity of the seed may appear. Whoever will reverently ponder this, will easily see that, in this interpretation, which is that of Paul, there is nothing tortuous or constrained.