Assurance is not a hall of mirrors for a believer

Critics of Reformed Theology, Calvinism, Federal Theology, etc. often have a problem understanding how a believer can have assurance they are saved.

Most Christians don't explain this. They can't or have never thought about it. It's simple, really:

Assurance is a riddle to the unregenerate. The fact is, to the regenerated believer 1) one has one's own life and experience to give one assurance. Before I was like that, I did that, I seemed to enjoy doing that, now I am like this, I don't do that, I don't enjoy doing that anymore. Other things one sees in oneself that are basic and easy to observe are things like 2) one's valuation for Scripture itself. I use to not value it. I use to mock it. Now I actually read it and meditate upon it and yearn to increase my understanding of it and to become 'mighty in the Scriptures.' Another mark of regeneration is 3) love for the brethren. This can seem ironic if you spend a lot of time in debate over doctrinal matters, but once you step out into the world and you recognize a true brother or sister in the faith you feel a real love for them. You and me against the world type of thing because it's usually in such conflict that one becomes aware of another person's belief.

That's a good, basic way to explain it to them.