Please explain in simple language the concept of the kingdom of God.
What Jesus means by the expression the kingdom of God is accepting God’s values as trustworthy and enough, not needing any “improvements,” as we often describe them.
I realize that I can sin by some action (for example, murder or theft), but I have never understood how someone can sin by inaction, by not doing something. How is that possible?
Yes, sins of commission (such as the examples you cited) are pretty easy to recognize.
As an 80-year-old Catholic, I was taught that missing Mass through your own fault is a mortal sin that, if unconfessed and unabsolved, could send you to hell forever. I know friends and other Catholics who are good people but no longer go to Mass.
How do I understand and find peace in the death of a parent?
Please accept my condolences on the death of your father or mother. Allow yourself to grieve properly but without being absolutely consumed or paralyzed by this death.