Or do you presume on the riches of his kindness and forbearance and patience, not knowing that God's kindness is meant to lead you to repentance?
When we presume, we make an assumption based upon our own thoughts, rather than taking in the sum of the evidence. For instance, we might presume that someone has good or bad intentions in an action, based on our previous experience with that person. Many times, if we do not have any previous experience with that person we might then base our presumptions on similar types of persons. This is called stereotyping, and while in some instances might be correct, it is unfair to judge someone by the acts of others.
In this verse, presumptions are being made about God. If I were God, I would not hold someone accountable for their mistakes, because I would be a kind and loving God. Or since God has shown me a kindness in this area, He will also show me kindness and grace in all areas.
I want to take a look at the word "forbearance." It carries the meaning of tolerance or self-restraint. So then God has shown us kindness and self-restraint and patience. If someone restrains themselves from doing something, can we expect that to continue no matter what? Even in turning the other cheek, do we believe that that is appropriate in every instance? When we restrain ourselves, we look past our desires so that we might help others for their ultimate good. I might restrain myself from telling someone what I really think in order not to hurt their feelings. But yet there are other times when it is appropriate to voice my feelings for their good. Or I might restrain myself in dealing with my children in some ways, but I will not restrain myself in dealing with them in every way.
God is kind, and He restrains Himself from punishing us when we deserve it. He does this because in His grace, He desires that we come to salvation in Christ where the price for that punishment has been paid. His purpose is not to condone or forget the offense, but rather that we repent and turn from that offense.
So don't make the presumption that God will continue to dismiss your sins, so you can just continue to live in them. That might be how we want it to work, but that is not how God works. God's kindness has a purpose.