At the center of the history of redemption is a cradle, a cross, and a crown.
All of the Old Testament, from creation to the fall of the first Adam, from the calling of Abraham and God's purposes for Israel, and including all the prophets, spoke in preparation and in foreshadowing of the Coming One, the last Adam, the God-man who would make all things right.
"When we could not come to him, he came to us." The truth of the incarnation is vital to the gospel: unless the Messiah would be both man and God he could not accomplish what he needed to accomplish. He must be infinitely strong to save (God) and he must be a perfect Substitute for us (fully human). Then his death could be a full atonement, a completely gracious redemption. We could not come to him unless he came to us.
This is the center of history: the life, death and resurrection of the Lord Jesus. An accomplished salvation at his initiative. A Savior in every sense of the word.
Now in the New Testament the Apostles proclaim that Jesus is risen and ascended with all authority. And they explain all that this means for us who believe. This gospel announcement is proclaimed now to all nations until Christ returns. And he will visibly establish his glorious reign in the new heavens and new earth.
Lack of understanding all these implications of the gospel leads to so many of our problems. Tim Keller writes,
Most of our problems in life come from a lack of proper orientation to the gospel. Pathologies in the church and sinful patterns in our individual lives ultimately stem from a failure to think through the deep implications of the gospel and to grasp and believe the gospel through and through. Put positively, the gospel transforms our hearts and our thinking and changes our approaches to absolutely everything. When the gospel is expounded and applied in its fullness in any church, that church will look unique. People will find in it an attractive, electrifying balance of moral conviction and compassion. D. A. Carson writes the following: The gospel is regularly presented not only as truth to be received and believed, but the very power of God to transform (see 1 Cor 2; 1 Thess 2:4; [Rom 1:16–17])… One of the most urgently needed things today is a careful treatment of how the gospel, biblically and richly understood, ought to shape everything we do in the local church, all of our ethics, all of our priorities. (--Tim Keller, Center Church)