Jamieson, Fausset and Brown on Deuteronomy 4:15

De 4:14-40. A Particular Dissuasive against Idolatry.

15. Take ... good heed ... for ye saw no manner of similitude--The extreme proneness of the Israelites to idolatry, from their position in the midst of surrounding nations already abandoned to its seductions, accounts for their attention being repeatedly drawn to the fact that God did not appear on Sinai in any visible form; and an earnest caution, founded on that remarkable circumstance, is given to beware, not only of making representations of false gods, but also any fancied representation of the true God.