Why didnt the 10 commandments say anything about blasphemy?
Blasphemy against God isn’t just in disrespectful behavior, or what one says; that’s only an outward expression of it. Blasphemy means “contempt,” and that comes from the heart. Not only do unbelievers or worshippers of other gods have contempt toward God and express it freely by verbally blaspheming, the world is also filled with hypocrites who act respectful, but inwardly they hold God in contempt and let the things of the world (“other gods”) take His place in their hearts. This too is blasphemy.