Blasphemy is denigration of our LordJesusChrist. Christians are about Christ Jesus and His HolySpirit. They aught not divert the glory, honour and praise due Christ Jesus to any other being.

Any person who does not glorify Christ Jesus as Mighty God, Everlasting Father, Prince of Peace, King of Kings and Lord of Lords is not truly Christian.

A synagogue is the Jewish equivalent of a Christian church. It is a place of worship, teaching and fellowship.
What would a synagogue of Satan look like? -> It would not lift up Jesus -> It would not lift of the WordOfTheLord. -> It would not lift up the HolySpirit. -> It would speak truth tainted with lies. -> It would have form of godliness but deny the power.