John Calvin on Genesis 28:21

21. Then shall the Lord be my God. In these words Jacob binds himself never to apostatize from the pure worship of the One God; for there is no doubt that he here comprises the sum of piety. But he may seem to promise what far exceeds his strength; for newness of life, spiritual righteousness, integrity of heart, and a holy regulation of the whole life, were not in his own power. I answer, when holy men vow those things which God requires of them, and which are due from them as acts of piety; they, at the same time, embrace what God promises concerning the remission of sins by the help of his Holy Spirit. Hence it follows that they ascribe nothing to their own strength; and also, that whatever falls short of entire perfection does not vitiate their worship, because God, mercifully and with paternal indulgence, pardons them.