Sforno on Exodus 32:27/ספורנו על שמות ל״ב:כ״ז

1עברו ושובו משער לשער; in order to secure atonement for those members of the people who had not actively participated in the sin but were guilty of not having protested the sin. By not now protesting the execution of the guilty ones either they proved deserving of being forgiven.