I would add the more obvious definition of debt here that would include rebellion against duty and rules. It is both. A life lived both by rules or against rules is sin or debt. Living by faith is living life in a relationship with Jesus Christ and others rather than duty and rules. It is sin.
Legalism is a vast wasteland.
It's a flourishing garden. We can only thrive in life through the surrender of ourselves to the Lord and one another. All of our spiritual debt it paid. The debit side of the ledger is cleared. Think of it this way.