The Torah was given with its signs — its built-in warnings against idolatry. The Mekhilta explains why this matters. Israel might have reasoned as follows: we are commanded against worshipping idols, but surely the prohibition applies only to public worship. If we hide the idols underground, in the recesses of the earth, we will not be held accountable.
The Torah anticipated this rationalization. (Isaiah 2:14) describes God's judgment reaching "all the high mountains and all the lofty hills" — and by extension, every hidden place in the earth. The prophet's imagery covers every possible location where an idol might be concealed: elevated places, low places, public spaces, and secret recesses.
The message is unambiguous: whether revealed or hidden, idolatry is forbidden. You cannot circumvent the prohibition through concealment. The idol buried in a cellar is as forbidden as the idol displayed on a hilltop.
The Mekhilta underscores this with a theological declaration: "only to the Lord alone, with no admixtures of idolatry." The exclusivity of God's worship does not depend on visibility. Private idol worship is not a lesser offense than public idol worship. The sin is in the worship itself, not in its display. God's prohibition penetrates every surface, reaches into every hidden chamber, and leaves no space — physical or psychological — where an idol can exist without violating the commandment.