http://revach.net/article.php?id=1675

Section: Parshas HaShavua   Category: Quick Vort
Parshas Bo: No Chometz and No Eigel
The Gemara in Avodah Zara (4) says that Rebbi Yehoshua Ben Levi says the Bnei Yisroel were not on such a low level that they would make an Eigel.  It was far beneath them to worship Avodah Zara.  They did it only to give future Baalei Teshuva hope that they too can repent regardless of the severity of their aveiros.

How does Rebbi Yehoshua Ben Levi know this?  Rav Sholom MiKaminka answers that it is brought down from the Arizal that anyone who is careful from even the slightest trace of Chometz on Pesach will be guaranteed not to do any aveira the entire year.  No less than the Torah itself testifies on the Bnei Yisroel (12:39) "Ki Lo Chometz", they ate only Matza and had no Chometz.  If so they could not have sinned a few months later so it must be that they were only giving hope to the Baalei Teshuva. (Iturei Torah)