Deuteronomy 9:4

4.Speak not thou in thine heart. He now more plainly warns the people not to exalt themselves in proud and foolish boasting. If they had not been naturally so depraved and malignant, it would have been sufficient to point out God’s grace in a single word; but he could not induce them to gratitude except by correcting and destroying their pride. He therefore takes away this stumblingblock, in order that God’s generosity might be conspicuous among them. “To speak in the heart” is equivalent to reflecting or conceiving an opinion. Wherefore Moses not only reproves the boasting of the lips, but that hidden arrogance, wherewith men are puffed up, when they take to themselves the praise which is due to God. Moreover, he not only prohibits them from ascribing it to their own valor, that they had routed their enemies, and gained possession of the land, but also from imagining that this was the just recompense of their merits. For God is not less defrauded of His glory when men oppose their righteousness to His liberality, than when they boast that whatever blessings they have are obtained by their own industry. To make this more ‘clear, I will repeat it. Moses does not forbid the people from thinking that they had themselves acquired the land without God’s aid; nay, he takes it for granted that they themselves will acknowledge that it was by God’s help that they were victorious; but he is not contented with this limited gratitude unless they at the same time acknowledge that they had deserved nothing of the kind, and therefore that it was a mere and gratuitous act of His bounty. The reason given in the second clause does not appear sufficiently (248) conclusive, viz., that the nations were driven out on account of their own wickedness; for it might have been that what God took away from these wicked reprobates He transferred to those who were more worthy; but. it appears to be an indirect admonition, that the Israelites should compare themselves with these nations; because it was evidently to be gathered by them from thence, (249) that they had not acquired this foreign land, from which the former inhabitants had been ejected, by their own righteousness. And this is still more clearly expressed in the two next verses.