Lest you are drawn after them, or lest you emulate them, or lest you do as they do, and they become a stumbling block to you.
"after their being destroyed before you": Why do I destroy them before you? Because of their deeds. That you not do as they do and they come after you and destroy you.
"and lest you inquire after their gods, saying, etc.": that you not say: Since they go out in a toga, I will go out in a toga, too; since they go out in purple, I will go out in purple too; since they go out in fancy headdress, so will I.