And they made a calf - This was made of the ear-rings and ornaments which they had brought from Egypt, Exodus 32:2-4. Stephen introduces this to remind them how prone the nation had been to reject God, and to walk in the ways of sin.
Related Commentaries of Acts 7:41
Acts 7:41
41 And they made a calf in those days, and offered sacrifice unto the idol, and rejoiced in the works of their own hands.