> What if one's parents have condescended and belittled them for decades?

It's all a matter of perspective. You see, one day, your parents will show up at your doorstep and expect you do the mature thing and take care of them in their old age. You could then go off on a long rant, wash all the dirty laundry, and finally slam the door in their face. Or you tell them "I forgive you" and close the door in their face.

That's the power of forgiveness.