If the prohibition against "gods of gold" addresses making extra cherubs beyond the commanded two, what does the additional prohibition against "gods of silver" teach? After all, the golden cherubs already establish the principle. Why mention silver at all?
The Mekhilta explains with a practical legal scenario. Throughout the Temple, there was a general principle: if a golden implement was unavailable, it could be made of silver instead. Gold was preferred, but silver was an acceptable substitute for most sacred vessels.
Someone might logically extend this principle to the cherubs. If there is no gold available, make the cherubs of silver instead — just as you would with other Temple implements. To prevent this, the Torah specifies "gods of silver." If the cherubs are made of silver instead of the commanded gold, they become "gods of silver" — idolatrous objects rather than sacred ones.
The cherubs were unique among Temple implements. For every other vessel, the substitution principle applied. For the cherubs, it did not. They had to be gold, precisely as commanded, or they crossed the line into forbidden images. This distinction underscores how sensitive the cherubs' status was. They were the only permitted representations of living beings in the entire sanctuary. That permission was narrowly defined — two figures, made of gold, positioned exactly as specified. Change any variable and the sacred became the profane.