Very early in His earthly ministry, Jesus started preaching about the kingdom of God. Matthew indicates, “From that time Jesus began to preach and to say, “Repent, for the kingdom of heaven is at hand” (4:17). It seems that the kingdom not only drew near during Jesus’ first advent, but actually arrived. Later in His ministry Jesus said, “But if I cast out demons by the Spirit of God, surely the kingdom of God has come upon you” (Matthew 12:28). The kingdom of God had come and people began to enter it by believing in Jesus and repenting of their sins (21:31, 23:13; cp. Acts 19:4). Paul wrote of people God has saved, “He has delivered us from the power of darkness and conveyed us into the kingdom of the Son of His love” (Colossians 1:13).
The kingdom of God has been inaugurated and Jesus is reigning as King at the right hand of God (1 Cor. 15:20-28; Heb 1:13; cp. Acts 2:32-36). However, the kingdom of God has not yet been consummated. The kingdom of God will be consummated at Christ’s second coming when He will establish it upon the earth. He will come in the glory of His Father (16:27), and judge the living and the dead. The righteous will enter into the eternal kingdom that is eternal life, but the unrighteous will go away to everlasting punishment (25:31-46).
Three disciples of Christ were permitted to see, by way of a vision, Jesus coming in His kingdom (16:28-17:13). This account is often called the mount of transfiguration. Jesus took Peter, James and John up on a mountain where His face shone like the sun, and His clothes became as white as the light. This brightness is symbolic of the glory of Christ in which He will come and reign upon the earth (See 16:27). This vision was in essence a prophecy of Christ’s second coming when He will reign in majesty. It was so vivid, Peter later wrote of being eyewitnesses of Jesus’ future coming and reign (2Pet. 1:16-18).

For we did not follow cunningly devised fables when we made known to you the power and coming of our Lord Jesus Christ, but were eyewitnesses of His majesty (2 Peter 1:16).