The gospel is the "good news" of the death, burial and resurrection of Christ (1 Corinthians 15:1-4) and is the power of God unto salvation to everyone that BELIEVES.. (Romans 1:16) To "believe" the gospel is to trust in the death, burial and resurrection of Christ as the ALL-sufficient means of our salvation.
The gospel is a message of grace that is to be received through faith. (Acts 15:7-9; 20:24; 26:18) The gospel is not a set of rituals to perform, a code of laws to be obeyed or a check list of good works (including water baptism) to accomplish as a prerequisite for salvation.
Before his death, Jesus told the twelve that he had to die and that he would rise on the third day. Looking back upon this passage we see this as Jesus speaking of an event that would change everything! However, it is unquestionable that the disciples were ignorant of its meaning at that time.
“Then he took unto him the twelve, and said unto them, Behold, we go up to Jerusalem...And they shall scourge him, and put him to death: and the third day he shall rise again. And they understood none of these things: and this saying was hid from them, neither knew they the things which were spoken. – Luke 18:31-34. hmm... Through progressive revelation, the further content of the gospel that came through a revelation of Jesus Christ to Paul (Galatians 1:11-12) at that same time was still a mystery.
After his crucifixion, instead of anxiously waiting his resurrection, the apostles were still ignorant, disheartened at the death of their redeemer, and did not even at first believe the testimony of his resurrection. (Mark 16:14, Luke 24:19-24) It was not until after his resurrection that Jesus appeared to the apostles and opened their understanding. (Luke 24:44-47)
Through progressive revelation, there is a more distinct element to the content of the gospel which Paul received from Christ and is called "the mystery of the gospel." (see Ephesians 6:19 and compare with Colossians 1:26-27; 4:3) This new revelation is that the Gentiles are fellow heirs and fellow members of the body, and fellow partakers of the promise. (Ephesians 3:6) Such equality, Jew and Gentile united together in one body was previously unknown. The distinctive message of the church is that Jew and Gentile alike may believe the gospel and be united together into one body. (Ephesians 1:13; 1 Corinthians 12:13)