Topics are mostly for categorizing content and it's main purpose is discovery. They do not have any permission or access settings attached to them.
To make content visible to specific users, or after they're logged in, you will need to use Groups instead which can be easily installed from the Tribe app store. You can set them as Private groups and then set default groups for your members which means all new members will be part of these groups and see the content on their feed. Additionally, you can add all existing members to these groups inside the group app.