During Passover, we read the following passage, from Exodus 12:12:
וְעָבַרְתִּי בְאֶרֶץ מִצְרַיִם בַּלַּיְלָה הַזֶּה, וְהִכֵּיתִי כָל בְּכוֹר בְּאֶרֶץ מִצְרַיִם מֵאָדָם וְעַד בְּהֵמָה וּבְכָל אֱלֹהֵי מִצְרַיִם אֶעֱשֶׂה שְׁפָטִים, אֲנִי יי.
And God said, "I will go through the land of Egypt on that night… and I will mete out justice against all the gods of Egypt. I the Eternal.
God Himself is saying that He will fight other gods. How can God fight other gods, if other gods don't exist?