וְהַקְּלָלָ֗ה אִם־לֹ֤א תִשְׁמְעוּ֙ אֶל־מִצְוֺת֙ יְהוָ֣ה אֱלֹֽהֵיכֶ֔ם וְסַרְתֶּ֣ם מִן־הַדֶּ֔רֶךְ אֲשֶׁ֧ר אָנֹכִ֛י מְצַוֶּ֥ה אֶתְכֶ֖ם הַיּ֑וֹם לָלֶ֗כֶת אַחֲרֵ֛י אֱלֹהִ֥ים אֲחֵרִ֖ים אֲשֶׁ֥ר לֹֽא־יְדַעְתֶּֽם׃ (ס)
咒詛，則是在你們不聽從永恆者你們的上帝的誡命，偏離我今日所吩咐你們的道路，去隨從你們向來不認識的別神時，臨到你們的。
Rashi
מן הדרך אשר אנכי מצוה אתכם היום ללכת וגו׳ [IF YE DEPART] FROM THE WAY WHICH I COMMAND YOU THIS DAY, TO GO [AFTER OTHER GODS] etc. — You thus learn that he who serves idols departs from the entire path of life that Israel has been commanded. From this passage they (the Rabbis) taught the well-known dictum that he who acknowledges the divinity of an idol is as though he denied the Torah in its entirety (Sifrei Devarim 54:4).
Or HaChaim
והקללה אם לא תשמעו, "And the curse in case you will fail to hearken, etc." Moses means that anyone who prevents himself from hearkening to the voice of G'd has already cursed himself. This is why the word והקללה appears before the words "if you will not hearken." Moses adds that in addition you will have demonstrated that you depart from the normal path of life by preparing for yourselves alternate but leaking water cisterns. In effect this is what the words: "you will depart from the way and follow alien deities" are all about.
Rabbeinu Bahya
אם לא תשמעו אל מצות ה' אלו-היכם וסרתם מן הדרך, “If you will not listen to the commandments of the Lord your G’d and you will deviate from the path, etc.” This verse teaches that anyone guilty of idolatry is as if he had abandoned all of G’d’s commandments. This verse also inspired the statement by our sages in the Sifri that anyone who acknowledges that idolatry has any substance to it is considered as having denied all aspects of Torah.