As a young Christian, I was taught that the gospel is a plan-"the plan of salvation." Some Bible teachers used to frame that plan into "Four Spiritual Laws" and "The Romans Road."
In the first-century Roman world, however, the word "gospel" was used to describe the announcement that a new emperor had taken the throne. "Heralds" would be "sent" throughout the Roman Empire to announce this "good news."
Their message was, "We have a new emperor. His name is Tiberius Caesar, adjust your life and bow the knee." Interestingly, the Roman emperor was also called "Savior" and "Lord" and was regarded as the one who would establish "peace" in the Empire.
In addition, the Roman emperor was expected to bring justice, peace, prosperity, and blessings to the world. He was also called "Pontifex Maximus" which means "chief priest." The Romans also believed that when an emperor ascended into heaven, he was enthroned as being divine. Thus the emperor (at his death) was also called "son of God."
Consequently, when the apostles ("sent ones") used the term "gospel" and declared that Jesus was now the Lord and Savior of the world, it was a direct affront to the Roman hierarchy, especially Caesar (see Acts 17:7, as an example). The believing Jews no doubt connected the gospel-preaching of the apostles to Isaiah's prophecy-a proclamation that God Himself was now reigning in the Person of Jesus (see Isa. 52:7).
If you examine everywhere the term "gospel" is used throughout the New Testament, you will discover that it's always bound up with the Person of Jesus. (His work is united with His Person. While people regularly separate His work from His Person, you can't separate His Person from His work. The same is true with His teachings. See Jesus Manifesto if you want a detailed discussion on this point.)
In His preaching and teaching, Jesus consistently pointed to Himself. Read the four gospels carefully sometime and count the number of times that Jesus speaks about Himself. You will have no doubts that His message-His gospel-was Himself. Paul, Peter, John, et al. preached the same gospel as did Jesus. Their message was also Christ.
In short, the message of the gospel is Jesus Christ as Lord (=world ruler), Savior, the fulfillment of the entire Old Testament (including the Adamic commission, the prophets, the priests, the kings, the sages, the temple, the sacrifices, the land, the Law, the promises, and the entire story of Israel), and Jesus as the Resurrection and the Life.
The gospel is also bound up with the eternal purpose of God in Christ-which is not separate from Jesus-or as Paul calls it, "the mystery." Romans 16:25, Ephesians 6:19 and Ephesians 3:7-11 associate the preaching of "the mystery" and "the unsearchable riches of Christ" with the gospel. This point is often missed among those who teach about the gospel today, for the eternal purpose ("the mystery") gets very little air-play in evangelical circles today-even though it's at the heart of New Testament revelation.
The gospel, then, isn't a postulate; it's a Person. Properly conceived, the gospel is the proclamation of Jesus-His Life, Story, and Work-reaching back from the Old Testament story of Adam, the patriarchs, and Israel to the New Testament which announces His first and second appearances.
Jesus of Nazareth is the good news.
For this reason, the four Gospels were regarded as "the gospel" by the early church. And what story do the four gospels tell? They tell the story of Jesus. He is the gospel incarnate.
While many modern Christians reduce the gospel to two verses in 1 Corinthians 15 (v. 3-4), Paul's "definition" of the gospel in that passage actually extends to verse 28 when God becomes "all in all." And the story found in the gospels is the same story that is told in the New Testament.
Charles Spurgeon got it right when he said, "Preach Christ . . . He is the whole gospel. His person, offices, and work must be our one great, all-comprehending theme."