Q&A: Which commandment was the most difficult for the Israelites to accept when they received the Torah at Mt. Sinai?

As the Book of Exodus itself suggests, it was the probably the second:

In modern Square Hebrew script: לא תעשה לך פסל וכל תמונה
Phonetically: Lo ta’aseh lekha pesel vekhol tmunah

—i.e. ‘Thou shalt not make unto thee any graven image, or any likeness’

We know this because this was the first transgression that the Israelites committed, while Moses was up on Mt. Sinai receiving the Ten Commandments, by creating and worshipping a ‘golden calf’ (in reality, a bull).

(In their defence, it should be pointed out that, since Moses hadn’t returned yet with the Commandments, they were technically unaware of it. But as every law enforcement official will tell you, ignorance of the law is no defence…)

As Canaanites—indeed, like everyone in the ancient world—the notion that Moses was trying to instil in them that God was an abstract entity with no material form was conceptually very difficult for the Israelites to accept. In fact, it was probably even more difficult than the first commandment:

(לא יהיה לך אלהים אחרים על פני— Lo yihyeh lekha elohim aḥerim al panai—‘Thou shalt have no other gods before me’) which doesn’t state that God is the only god (that was probably too radical a concept to ask of the nation at this stage of its history), but merely that He must be worshipped before all others.

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(Originally written in reply to a question at Quora.com)