John Calvin Commentary Deuteronomy 4:39

"Know therefore this day, and lay it to thy heart, that Jehovah he is God in heaven above and upon the earth beneath; there is none else." — Deuteronomy 4:39 (ASV)

Know therefore this day. He again stresses what we have recently discussed: that the glory of the one true God was proved by the miracles, but he does so by way of exhortation. For he desires them to consider carefully and attentively what God had shown them, because in so plain a matter there would be no excuse for error or ignorance.

He therefore infers from what has been said before that the people must beware of shutting their eyes to the clear revelation of God’s power, and so he urges them to keep it in memory, because human ingratitude is all too prone to forgetfulness. He then reminds them why God wished to be known: namely, that they might keep His Law and obey His statutes.

In summary, they would be inexcusable if they did not obediently receive the Law, which they knew to have come from God. For they would surely be worse than stupid if the majesty of God, known and understood through so many proofs, did not awaken them to reverence. And so that they would not undervalue the doctrine as coming from a mortal man, he expressly acknowledges that he is indeed the minister, yet he had set nothing before them that he had not received from God.