How do you know at what instant in time you became unsaved again or
saved again and how do you know if you are or aren't now?

This sounds like works oriented theology.
You're constantly working to make sure you're saved all the time not knowing.

Four types of people.
1. Are not saved and know it.

2. Are not saved but think they are.

3. Are saved and know it.

4. Are saved but think they are not.