So then, brothers, we are debtors, not to the flesh, to live according to the flesh.
Romans 8:12 ESV

We are debtors because God gave us life when we could not have given it to ourselves. We don’t owe the flesh anything in regard to our salvation, because it is unable to do anything in our favor. God enables us to live in the spirit even while our flesh continuously tries to drag us back into the grave.

This is a debt that we can never repay because it is of infinite worth. The only thing we can do to repay God’s gift is to live as God instructs us to live, according to his commandments as disciples of Yeshua, and making new disciples along the way.