Are Other Religions Different Expressions of the Same God?

Questions about whether other religions are different expressions of the same God, and how to reconcile the idea that people who have never heard the gospel are guaranteed to go to Hell with the fact that God is a good judge who shows mercy on whom he chooses to show mercy.

• Are other religions different expressions of the same God? If the Bible centers on one region, how did isolated people come to know him? Could their native religions have been another way God revealed himself?
• How do you reconcile the idea that people who have never heard the gospel are guaranteed to go to Hell (per Romans 10) with knowing God is a good judge who shows mercy on whom he chooses to show mercy?