What would that mean to a first century public?
In the first century Jewish thought, the idea had already been formed that there will be a coming Messianic kingdom.
Jesus’ teaching in Mark is apocalyptic: “The time has been fulfilled” (Mark 1.15) implies that this current evil age, seen on a time line, is almost over. The end is almost within sight. “The Kingdom of God is near” (Mark 1.15) means that God will soon intervene in this age and overthrow its wicked powers and the kingdoms they support, such as Rome, and establish his own kingdom, a kingdom of truth, peace, and justice. “Repent and believe the good news” (Mark 1.15) means that people need to prepare for this coming kingdom by changing their lives, beginning to align themselves with the forces of good instead of the forces of evil, and by accepting Jesus’ teaching that it was soon to happen. [1]In modern days the word apocalypse is loaded with images of total earthly destruction, but the word it self comes from the greek word 'αποκαλυψις' and means to reveal something. But what is it to be revealed?
...we misunderstand ancient Jewish and/or Christian eschatology if we think it was about the end of the world, if we think it was about the divine destruction of this physical earth. In the King James Version of the Bible, the phrase “end of the world” is repeated in (Matthew 13.39,49) and in chapter (Matthew 24.3,20). But the Greek term translated there as “world” is actually 'αιων', from which we get our word eon, meaning a period, a time, an era. What is to end is this present “era” of evil and injustice, suffering and oppression. ... For ancient Jews and Christians,only God could destroy the earth, but God would never do that. Why? Because six times during creation God declared the product “good,” and when all was finished,“God saw everything ... was very good” (Genesis 1:31)... therefore, God would never destroy God’s own creation. [2]
To the Jews of the first century, the reference to the coming kingdom of god would have certainly brought the promised Messianic kingdom in mind, which was expected to endure for ever (Daniel 2.44). The coming wrath is that of god, against those that did not align them with the forces of good.
The kingdom of god was certainly not about a transfer of recently destroyed earth to heavenly realms. It was about rejuvenating the earth, transforming it into peaceful place.
... for example, for Jesus in the Lord’s Prayer the Kingdom of God is about the will of God “on earth as in heaven.”The original mock-up for God’s earthly kingdom has been retained in heaven—like the model in an architect’s office—but the final construction site will be on the earth itself. [3]
see also (Isaiah 11.6–9)
According to Mark and Matthew, John and Jesus were preaching the imminence of the coming of the kingdom of god. People had to repent and make their paths straight, otherwise they will experience the wrath of god. At least Matthew thought that god's forgiveness was restrained from the religious elite.
