Musar su Genesi 28:15

Even though a person be guiltless and completely righteous he should always be fearful that a sin might come to him. And so have we found in the case of Jacob that even though the Holy One, Blessed be He, had said to him "And I will keep Thee withersoever thou goest" (Gen. 28:15), Jacob was nevertheless afraid that he had sinned (and lost the divine promise). And so did David say "If I had not believed that I would see the goodness of the Lord, in the land of the living!" (Ps. 27:13). Even though David knew that he was a perfectly pious man he feared that sin might prevent him from receiving the pleasant reward of the righteous (Berakoth 4a).

"He was afraid and distressed;" the former was due to Jacob's fear that the accuser in the Celestial Regions might have come across sins committed by Jacob that he had not been aware of. He was also afraid that Esau's merit in having performed the commandment of honouring father and mother during all the years Jacob had not performed it might now support him. We find that Moses too was afraid before engaging Og, King of Bashan, in battle, fearing that the merit of Abraham might assist Og, as explained by Rashi on Numbers 21,34. [The merit of Og referred to must be that he became instrumental in Abraham saving Lot, by having told him that the latter had been taken prisoner, even though that had hardly been Og's motive, as Rashi himself explains on Genesis 14, 13. Ed.] Bereshit Rabbah 76, 1, quoting Rabbi Pinchas, understands our verse in a similar vein. "There were two people who had received specific assurances from G–d, and still they were reported as being afraid. One is the choicest of the patriarchs, Jacob, to whom G–d had said: "I shall be with you;" in the end we nonetheless find him afraid of his upcoming encounter with Esau, seeing the Torah says: ויירא יעקב. The other person was the choicest of the prophets, Moses. G–d had told him "for I shall be with you" (Exodus 3,12). Still, we find G–d had to tell him in Numbers 21: "Do not be afraid of him (Og)." G–d certainly would not have bothered to reassure Moses had he not been afraid, (though the Torah did not report this as a fact). Thus far the Midrash.

Nachmanides writes that it is possible that when speaking about events in the future, one should do so in a conditional tense. He quotes several examples. It does not mean that Jacob entertained ambivalent thoughts. It simply means that when the time comes when the condition applies the vow will become operative. We still must understand why Jacob used an expression that lends itself to more than one interpretation, especially where such a sacred subject is involved.