In the postmillennial view, God’s covenant with His chosen people is regarded as one consistent covenant that was more fully revealed from the time of Abraham to the time of David and onward to the time of Christ, when the new covenant of grace in His blood was revealed. We see the kingdom of heaven as a kingdom that Jesus brought to earth with Him, one that has always been prophesied to start small and grow into something enormous here on Earth.
Many Christians today see the Kingdom as something that won’t be seen on Earth; we must wait until this earth passes away. But a great deal of scripture suggests strongly that the old has already passed away and that the new has come. The kingdom of God is here and now, and we have only to realize it.
God promised in Genesis 8:21 to never again curse the ground or destroy all living creatures. Surely He didn’t reserve some other means of destruction in that promise! This is part of God’s covenant with His people (Genesis 9:15). When Jesus said heaven and earth will pass away, He didn’t say they would be physically annihilated. We view this rather as a total transformation into something completely new. We see passages quoted by futurists in support of Earth's destruction as being more figurative rather than absolutely literal.
There are many scriptures that predict an exponential growth of God’s kingdom on earth. Here are a few of my favorites:
Genesis 12:3: [To Abram] “…and all peoples on earth will be blessed through you.” (Galatians 3:8 interprets this as a prophecy being fulfilled on Earth.)
Psalm 72:7-11: “In his days may the righteous flourish and prosperity abound till the moon is no more. May he rule from sea to sea and from the River to the ends of the earth. May the desert tribes bow before him and his enemies lick the dust. May the kings of Tarshish and of distant shores bring tribute to him. May the kings of Sheba and Seba present him gifts. May all kings bow down to him and all nations serve him.”
And verse 17: “May his name endure forever; may it continue as long as the sun. Then all nations will be blessed through him, and they will call him blessed.”
Daniel 2:35: “But the rock that struck the statue became a huge mountain and filled the whole earth.”
And verse 44: “In the time of those kings, the God of heaven will set up a kingdom that will never be destroyed, nor will it be left to another people. It will crush all those kingdoms and bring them to an end, but it will itself endure forever.”
Matthew 13:31-32: “The kingdom of heaven is like a mustard seed, which a man took and planted in his field. Though it is the smallest of all seeds, yet when it grows, it is the largest of garden plants and becomes a tree, so that the birds come and perch in its branches.”
Luke 13:20: “What shall I compare the kingdom of God to? It is like yeast that a woman took and mixed into about sixty pounds of flour until it worked all through the dough.”
Jesus did say the way was narrow, but He also said, “…many will come from the east and the west, and will take their places at the feast with Abraham, Isaac and Jacob in the kingdom of heaven” (Matthew 8:11). The gate was small for the Jews He was speaking to at that time, few of whom were saved. But things changed after He died and rose again and His message was carried around the world. Paul announced in Colossians 1:6 that “the gospel is bearing fruit and growing throughout the whole world.” That sounds like a lot of people to fit through a narrow gate!
May God bless you and cause you to grow in your understanding of His word!