The kingdom of God is a life-task, a spurring into true purpose.
And yet a person can certainly have a purpose in life, or a purpose from week to week, even day by day, a good purpose, without having first committed themselves to the kingdom of God. But where do these purposes lead? To accomplishment or failure, to achievement or continued striving. But then upon meeting your purpose, where does it lead? What then?
A purpose is a feeling of purpose, but I said true purpose. What is true purpose? A true purpose does not end with the end. A person who possesses true purpose also has smaller daily, weekly, yearly rounds of goal-setting and striving. But true purpose you were born with. True purpose comes from the outside. Or did you create yourself? Did you decide on how your neurons were laid out or your physiological temperament? Did you decide on what your face would look like? Did you decide all of your future travails? Did you decide how long you would live, and what tragedies would befall you?
True purpose accounts for the cosmic environment into which you are born. You can certainly make choices about your identity, and your decisions affect who you are and how people perceive you. But you can only say that you created yourself if you also first created the entire natural world. And so you find yourself stranded in orders of nature and chance far beyond your control. How do you orient yourself, so that your purposes are not at the complete whim of chance and fortune? You do that by identifying what exactly you have control over. And when you find that, you find true purpose, because that is the only thing that is completely up to you.
And we can take that very abstract idea about the core of the self, and objectify and spatialize it, so that we can talk about it at all. And that useful form for communication, discussion, and upbuilding, that concept clarifies the aim of true purpose. And that concept, that reality (because can reality ultimately be anything more than a concept?) is called the kingdom of God.