The kingdom of God is the prevalence and dominion of righteousness. It means the supremacy of truth, love, and peace. When the passage says, "His righteousness," it is not referring to the imputed righteousness of Christ, but rather to the personal righteousness of the subject of Christ's Kingdom of Grace. It is that quality of character which the law of the kingdom demands, which the Word instills in us, which the gracious operations of the Holy Spirit enables us to attain,—the conformity of the believer to the image of Christ. We are being encouraged to strive after personal piety, as becomes citizens of the Kingdom.
Seeking the Kingdom first does not mean that it is to be the first of a series of things which, when attained, gives place to the pursuit of something else. While the idea of order is not necessarily excluded, what is being emphasized is the idea of importance, priority, or supremacy. If this is done the order will fall in its proper place. To seek first the Kingdom of God is to make it the one thing for which we live. It is to make conformity to, and maintenance of righteousness in our life the supreme consideration in determining everything we do. It is the only thing that concerns our lives about which we have the right to be anxious, and the one thing about which we should be primarily anxious.
We are all too prone to regard the seeking of the kingdom of God as an tedious chore. Perhaps this will account for its not receiving the place in our lives that it should. But we ought to regard it with pleasure. A loyal citizen of an earthly kingdom loves to cherish the prestige of his country's honor, and promote her glory. He counts this a privilege. He does it with enthusiasm. He feels that his honor and his welfare are bound up in the prosperity of his country. If we would lay to heart the truth that our spiritual and eternal welfare is bound up in the reign of righteousness in our lives, if we properly appreciated the benefits and honors of heavenly citizenship, if we realized that God is a fountain of blessedness, and the more we possessed of Him, and the more thoroughly we permitted His kingdom to possess us. the happier we would be, it would inspire us to seek His righteousness with more cheerfulness and liveliness than we often display.