The “things on earth” does not mean things of an earthly nature, but things which transpire on earth, including redemption, regeneration, etc. The “things in heaven” also does not mean things of a heavenly nature, but things which transpire in heaven. In the following verse, the Lord said that He is the One who descended out of heaven and who is still in heaven. This indicates that He knows the things which transpire in heaven, because He is the One who is in heaven all the time. At the same time He is on earth telling people the things on earth.
Lord said, “Truly, truly, I say to you, unless a man is born anew, he cannot see the kingdom of God,” and in verse 5 He said, “Truly, truly, I say to you, unless a man is born of water and the Spirit, he cannot enter into the kingdom of God.” The Lord’s words here are a clear revelation that regeneration is the unique entrance into the kingdom of God. In order to enter into the kingdom of God, we need to be born again. There is no other way by which we can enter into the kingdom of God. The kingdom of God is the reign of God. It is a divine realm to be entered into, a realm which requires the divine life. Only the divine life can realize the divine things. Hence, to see or to enter into the kingdom of God requires regeneration with the divine life.
A kingdom is always related to life. The vegetable kingdom is related to the vegetable life, and the animal kingdom is related to the animal life. If you want to share in a certain kind of kingdom, you first need the life of that kingdom. Only birds can partake of the bird kingdom because only they have the life of a bird. Likewise, only men can participate in the human kingdom because only they have a human life. So, without the life of God, how could we ever share the kingdom of God?