When we are offered something that we can't pay for, what should our response be?
Trying to earn our own salvation is like trying to pay a mortgage on a house that is already fully paid off.
Even after Jesus paid the debt for our sins, there is still something we owe.
A good relationship with God isn't about collecting enough good deeds to pay for miscellaneous "bills" we have incurred. There is a great cost for us to be redeemed, but it has already been covered.
Although we may be tempted to keep score in relationships, making sure that we provide others with similar value to what they give us, the Bible has a different mission for us.