How is it possible that the Jews, after having rejected the sure foundation, can ever build upon it?

Jacob 4:17 And now, my beloved, how is it possible that these, after having rejected the sure foundation, can ever build upon it, that it may become the head of their corner?

 Jacob 4:18 Behold, my beloved brethren, I will unfold this mystery unto you; if I do not, by any means, get shaken from my firmness in the Spirit, and stumble because of my over anxiety for you.

Jacob says he will explain this, as long as he does not stumble.  He then tells the allegory of the olive tree.  He explains,

Jacob 6:5 Wherefore, my beloved brethren, I beseech of you in words of soberness that ye would repent, and come with full purpose of heart, and cleave unto God as he cleaveth unto you. And while his arm of mercy is extended towards you in the light of the day, harden not your hearts.

He doesn't speak again about those who rejected the foundation stone, but he speaks to his bretheren and asks them to not harden their hearts and accept Christ.

Jacob 6:8 Behold, will ye reject these words? Will ye reject the words of the prophets; and will ye reject all the words which have been spoken concerning Christ, after so many have spoken concerning him; and deny the good word of Christ, and the power of God, and the gift of the Holy Ghost, and quench the Holy Spirit, and make a mock of the great plan of redemption, which hath been laid for you?

So I think he did get shaken from his firmness (he must have sensed he would.)  He did however show how everyone needs to not harden their hearts, and not reject Christ, but he neglected to relate it back to the Jews.  Perhaps he wanted to avoid pointing a finger directly at the Jews, and perhaps he was told by the Lord to explain that everyone, not just the Jews, need to accept the foundation stone.