The Lord lightened David’s darkness. He does the same for us. Our Lord God is continually merciful to us by forgiving us our sins and He strengthens us with His Word so that our feet are, “like the feet of a deer.”
The kingdom which Jesus brings and which lasts forever is not an earthly kingdom. It is a heavenly kingdom and rule; it is the Church. The Church is the kingdom of Christ which cannot be shaken, eternal in the heavens.