Owe nothing to anyone except to love one another; for he who loves his neighbor has fulfilled [the] law.
I have often heard it said that there is no place in The Bible that tells me not to incur debts. I think that this verse is that place. In the verse immediately preceding this, Paul tells the believer to give everyone their due — taxes and custom and fear and honor are called out specifically. Then Paul comes to this: owe nothing to anyone except … love. I should, if I have debts, pay them. There is one debt which I will still owe after I have paid it, viz. love. The entire Law is fulfilled in loving God and others. Paul says it. More importantly, Jesus says it.
How, then, do I apply this?
I think the verse is self-explanatory. I should leave no debt unpaid and I should recognize that I have a debt of love that I owe to God and my neighbors that will never be paid in full. No matter how much I love God and my neighbor, I will still owe them both more love.