סָ֣רוּ מַהֵ֗ר מִן־הַדֶּ֙רֶךְ֙ אֲשֶׁ֣ר צִוִּיתִ֔ם עָשׂ֣וּ לָהֶ֔ם עֵ֖גֶל מַסֵּכָ֑ה וַיִּשְׁתַּֽחֲווּ־לוֹ֙ וַיִּזְבְּחוּ־ל֔וֹ וַיֹּ֣אמְר֔וּ אֵ֤לֶּה אֱלֹהֶ֙יךָ֙ יִשְׂרָאֵ֔ל אֲשֶׁ֥ר הֶֽעֱל֖וּךָ מֵאֶ֥רֶץ מִצְרָֽיִם׃
Zboczyli rychło z drogi, którą wskazałem im, zrobili sobie cielca litego, i kłaniali się mu i ofiarowali mu, i rzekli: Oto są bogi twoje Israelu, które cię wywiodły z ziemi Micraim!"
Chizkuni
סרו מהר, “they deviated quickly, etc.” they already elected a new leader for themselves instead of you whom I had appointed to be their leader. Not only that, but they also made a molten calf for themselves, contravening My command not to make themselves any cast images. My commandment included the prohibition to make such a cast image even if it was intended to symbolize Me. Although they had no idolatrous intention with what they did, they contravened My commandment.
Sforno
סרו מהר, they have strayed very quickly, etc. I have not even been able to complete the gift of the Torah I had wanted to give to you. A reference to “the Torah and the Mitzvah” in Exodus 24,12.
Or HaChaim
סרו מהר מן הדרך, "they have quickly departed from the path, etc." The meaning of these words is clear in view of the statement in Chulin 4 that if someone acknowledges even passively that there is some substance to idolatry he is considered as having denied the entire Torah. This is why G'd said: "which I have commanded them," i.e. "all that I have commanded them."