Note: this is an excerpt from the book It Must Be Finished.
The return of Jesus is part of the gospel. If we only speak about Jesus’ first coming and never mention His second coming, our gospel message is incomplete. This does not mean, every time the gospel is preached, we must talk about the return of Jesus, nor does it mean that speaking only about Jesus’ first coming is ineffective. On the contrary, every part of the gospel is glorious. However, the first coming without the second coming is not the full gospel.
Biblically, Jesus’ death on the cross did not fulfill all the promises of God; it secured them.Jesus’ return brings the fulfillment of promises secured at the cross—promises that include the resurrection from the dead, the judgment of the wicked, and the restoration of creation.
Because the first coming did not fulfill all the promises of God, the gospel is not just what Jesus has done; it is also what Jesus will do.
Well-known pastor and theologian John Piper summarizes the hope of the gospel this way:
Biblical hope is not finger-crossing. It is a confident expectation of good things to come. . . . We set our hope on the second coming of our Lord. . . . The second coming of Christ is the completion of his saving work. If you take it away, the whole fabric of his saving work unravels.
The apostle Paul also said all of creation is groaning for God to fulfill His promises at Jesus’ return:
For the creation waits with eager longing for the revealing of the sons of God. . . . For we know that the whole creation has been groaning together in the pains of childbirth until now. And not only the creation, but we ourselves, who have the firstfruits of the Spirit, groan inwardly as we wait eagerly for adoption as sons, the redemption of our bodies. (Romans 8:19, 22–23)
The New Testament gospel is a forward-looking message that sets our hope on the return of Jesus and everything that surrounds and accompanies that return.
Accordingly when the apostles preached the gospel, they spoke clearly about the second coming:
“The times of ignorance God overlooked, but now he commands all people everywhere to repent, because he has fixed a day on which he will judge the world in righteousness by a man whom he has appointed; and of this he has given assurance to all by raising him from the dead.” (Acts 17:30–31 ESV)
For the grace of God has appeared, bringing salvation for all people, training us to renounce ungodliness and worldly passions, and to live self-controlled, upright, and godly lives in the present age, waiting for our blessed hope, the appearing of the glory of our great God and Savior Jesus Christ. (Titus 2:11–13 ESV)
The return of Jesus was the greatest hope of the New Testament churchand her motivation towards holy living.The New Testament apostles preached the gospel this way as an example for the church in succeeding generations. We must follow their example and set our hope where the New Testament church set it.
 John Piper, “Our Hope: The Appearing of Jesus Christ,” Desiring God, May 18, 1986. Accessed April 14, 2017. http://www.desiringgod.org/messages/our-hope-the-appearing-of-jesus-christ.
 See Acts 1:11; Acts 28:20; Romans 8:18–25; 1 Corinthians 15:19; Galatians 5:5; 1 Thessalonians 2:19; Titus 2:13; Hebrews 10:37; 1 Peter 1:13; 2 Peter 3:4–14; 1 John 3:2–3.
 See 2 Peter 3:11–14, 1 Timothy 6:14–16; 2 Timothy 1:12.