Idolatry even at Sinai

 Tehilim (78:36-37) 36. Nevertheless they flattered him with their mouth, and they lied to him with their tongues. 37. For their heart was not constant with him, nor were they faithful in his covenant.

Shemos Rabbah (42:8):Even while they were standing at Sinai itself - proclaiming “We will do and obey” - their hearts were intent on idol worship.: They spoke fraudulently with their mouths and lied to Him with their tongues …

Shir HaShirim Rabbah (07:08) Rabbi Berekhya said in the name of Rabbi Levi: In two places, Israel acted deceitfully with G-d. At Sinai, they acted with their mouths but did not act with their heart, as it is stated: “But they beguiled Him with their mouth and lied to Him with their tongue…their heart was not steadfast toward Him”. In Babylon, they acted with their heart but did not act with their mouth, as it is stated: “For He does not afflict willingly”, as it is stated: “A man who is an adversary and an enemy. This wicked Haman”, and made their wound more painful.

Meiri (Avos Introduction): There is a strong emphasis and constant warning in the Torah to stay far away from idol worship since there has always been a strong attachment to it generation after generation. An intelligent person would naturally ask why this error of idol worship has repeated itself generation after generation? The answer is that the average person more readily believes that which he can perceive and comprehend that which is intangible and abstract. Therefore, a person’s nature draws him to idol worship.