Jesus said to Nicodemus, “If you don’t believe Me when I tell you about things that occur on earth, how will you believe Me when I tell you about things that occur in heaven?” Nicodemus had lived a while on the earth; he was a mature man. He was also a wise man. He knew how things worked, and was wise to the ways of the world. Therefore, when Jesus would tell him about earthly things, he had some basis for testing (at least in his mind) what Jesus said before accepting it. When it came to heavenly things, however, Nicodemus had never been there and had no understanding of how the place operated. Therefore, he would have to take entirely on faith anything Jesus said to him about it. Summarizing Jesus’ point to Nicodemus, believe the earthly things and it will give you a basis for believing the heavenly things; disbelieve the earthly things and you will never trust (and therefore never “see”) the heavenly things.
The apostles demonstrate a positive response to what Jesus described. They believed Jesus about the earthly things. Therefore, they had the faith to accept Him on the heavenly things. For example, the fact that Jesus ascended into heaven and sat down at the right hand of God is a heavenly thing that the apostles had no way of corroborating from their own experience. How would they know whether He sat on the left, right, behind, or somewhere else altogether? Because Jesus had proven Himself faithful, however, on so many earthly things, they had no problem accepting that final piece of testimony about His destination. Specifically, because they had seen for themselves that He had been born from the dead according to Psalm 2, it was quite natural for them to accept the testimony from Psalm 110 about where Jesus went once back in heaven.
We ourselves encounter the same dynamic when it comes to understanding the heavenly aspects of the coming of the kingdom of God (which can also be called the coming of the kingdom of heaven, Jesus’ Second Coming, the day of the Lord, Judgment Day, and so on). These heavenly things are described throughout the Scriptures, including the Old Testament prophets, the prophecies of our Lord, the references by the apostles, and the descriptions in the book of Revelation. If we don’t accept the earthly aspects of Jesus’ salvation by faith, we will have no basis to accept all these testimonies of heavenly things.
If someone does not truly walk by faith with Christ in the earth, they have a hard time believing that the kingdom has already come. They are fleshly minded and are looking for fulfillment of these scriptures in physical form. There are many such fleshly people in the earth today. They do not walk by faith; they walk by sight. Their trust is not in an invisible Lord but rather in a visible church of people. They are on the wrong side of the classic dichotomy of fearing man versus fearing God.
We have ample reason to believe all the earthly aspects of our salvation. Therefore, we should find it easy to take on faith the heavenly realities, such as the new heavens and new earth described in Isaiah and Revelation. Yes, we in fact live in what the book of Hebrews called “the age to come” that was at that time “drawing near.” Because we believe the prophets, the Lord, and the apostles about the when (which is an earthly thing we can understand), we can believe them about the what (which are heavenly things we have to take entirely on faith).