Start by reading Matthew 24:30-31 and Acts 17:30-31.
The second coming of Christ is the same as the final establishment of God's kingdom in this world. As it is written, “afterward they that are Christ's at his coming. Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power” (1 Corinthians 15:23-24). Jesus is “coming with His kingdom” (Matthew 16:28) and to put an end to all other kingdoms simultaneously.
When Christ comes again, He is coming to judge the world. This judgment is described this way: “In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ: Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power” (2 Thessalonians 1:8-9). And again, speaking of the final coming of God's kingdom, “and thy wrath is come, and the time of the dead, that they should be judged, and that thou shouldest give reward unto thy servants the prophets, and to the saints, and them that fear thy name, small and great; and shouldest destroy them which destroy the earth” (Revelation 11:18).
In order to preach the gospel and win the lost, properly, you must know that Christ is coming with vengeance, not grace. Grace is for now. Judgment comes soon. The arrival of the kingdom of God is the same as the arrival of the Second Coming of Christ and the great Day of Judgment. In this sense, the coming of God's kingdom is not a soft message, but a hard one. The expectation of the kingdom of God is meant to move the hearts of men with godly fear.