If "grace" is held out as a state of affairs - God saves believers apart from their works - then faith becomes an appreciation of a doctrine - I trust that God saves people through faith not works.
What then is assurance? It becomes something like - God has saved me because I have trusted the doctrine of grace. Where then is my confidence? It lies in that phrase "I have trusted." I'm having faith in my own faith.
But what if grace was specifically the Person of Jesus freely held out in the gospel? Well then faith is a receiving of Him. And if I have Him my confidence does not come from me at all. I don't trust my faith, I trust Jesus. And in Him I have full assurance.
In the impersonal state of affairs I build my assurance on having 'true faith'. But where will I find 'true faith'? In me?
When I know Jesus as my salvation then my assurance is based on having Him. How do I verify this? I look away to Christ clothed in the gospel - "Come to me and I will give you rest." My assurance therefore depends on His faithfulness (not on my flimsy faith).