Charles Ellicott 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 ... and consider. —“Consider,” i.e., reckon (the word for “impute” and “account” in Saint Paul’s argument to the Romans). Do not indulge any polytheistic notions regarding the Deity.

To us there is but One God. If every nation has its separate deity, how is it that Jehovah controls them all? His various dealings with Egyptians, Moabites, Ammonites, Edomites, Amorites, as well as with Israelites and Canaanites, mark Him as Lord of all.

There is none else. There are no more gods; if you desire to leave Him behind, there is no one else to serve. Compare Isaiah 44:8: Is there a God beside me? yea, there is no Rock. I know not any.