Jesus used this parable (Matthew 18:23-35) to teach His disciples the importance of forgiveness in God's kingdom. His intriguing narrative was designed to embed the principle into the hearts of the trainee apostles. If they were to be competent to lead the church, they had to learn God's strategy of forgiveness. It was not natural to them. For example, James and John were quick to condemn when Jesus wanted people to have time to repent and be forgiven (Luke 9:52-56).
 
In the parable, the servant's debt to his master was huge (ten thousand bags of gold). It would be impossible to pay off the debt. The only way that the master could get some of what belonged to him was to sell the family into lifelong slavery. But the servant knew his master's compassionate heart, admitted that it was his fault and pleaded for mercy. He was willing to try to repay everything. But the master knew he could not. Instead, he offered mercy and grace. Mercy in reprieving the servant from slavery was mixed with grace when the whole debt was cancelled (or forgiven).
 
Jesus often used financial debt as a way of talking about unforgiven sin. Without forgiveness no sinner can ever enter His kingdom. But without repentance they cannot be forgiven. So the first priority of the gospel (because it is God's priority) is to lead people to 'settle their accounts/debts with God'. Although the parable goes on to show that this servant was unwilling to forgive others and so had not really learned his lesson, Jesus first established the principle that God's heart is full of compassion and very kind towards those who plead for mercy.
 
The gospel is not about what we can do for God but about receiving what He has done for us. Yes, there will be a duty of thanksgiving, but we can never earn our salvation, nor can we do anything to atone for our sins or in any way make us acceptable to God. Those truths are the reason for the gospel - proclaiming that our sins were paid for by Christ even before we knew of His generosity. Calvary established the basis on which forgiveness could be both just and free. Just because Jesus has satisfied God's law which demands that sin must be paid for: and free because there is nothing we can do to earn the cancellation of our debts to God. Today is a good day to plead for mercy in the confidence that God will forgive and wipe the debt of sin away. It is also a good day to share that gospel news with someone else!