John Calvin Commentary Deuteronomy 4:35

"Unto thee it was showed, that thou mightest know that Jehovah he is God; there is none else besides him." — Deuteronomy 4:35 (ASV)

Unto thee it was shewed. He first says that God had so proved His divinity by miracles and wonders, that the Israelites might know certainly that He was God. From this, he also concludes that He is the only God. For although God’s holy name is torn in pieces by various opinions, while each one manufactures his own gods for himself, yet it is still sure that the power and dominion of God cannot be withdrawn from Him, but reside in a single subject, as the logicians say.

Therefore, the essence of the one God overthrows and annihilates all the other deities which we foolishly invent for ourselves. And this we must carefully note, for this has been the common error of all ages: to seek a mixture of many gods, whereas all these imaginations should vanish before the brightness of the true God.

In the following verse, he confirms this declaration, because God instructed His people out of heaven and in the fire. However, if it is asked how these two points agree, which seem to be opposed to each other—that God’s voice was heard from heaven and from the midst of the fire—I reply that Moses simply means that the voice which flowed out of the fire into the people’s ears was distinguished by plain indications which proved it to be heavenly.