Final answer:
The architectural device used to restrict access in the Jerusalem Temple and separate Gentiles from Israelites is called a templon. This barrier was widespread in Byzantium and can still be found in Orthodox churches today.
Step-by-step explanation:
The architectural device used to restrict access in the Jerusalem Temple and separate Gentiles from Israelites is called a templon. It was a sanctuary barrier consisting of chancel slabs, colonnettes, and an epistyle or templon beam. This architectural feature was widespread in Byzantium during that time and can still be found in Orthodox churches today, known as an "iconostasis."