The Bible tells us what the gospel is in I Corinthians 15:1-4 (KJV). "Moreover, brethren, I declare unto you the gospel which I preached unto you which also ye (you) have received, and wherein ye (you) stand; by which also ye (you) are saved, if ye (you) keep in memory what I preached unto you -- unless ye (you) have believed in vain. For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the Scriptures; and that He was buried, and that He rose again the third day according to the Scriptures."
According to I Corinthians 15:1-4, the gospel is the Good News about the death, burial, and resurrection of Jesus Christ. He died for our sins.
Watch Video: "The Gospel is the Power for God to Salvation" Workbook (a study on faith): "What Kind of Faith Do You Have?" Free Home Study Courses: (View, download, and print lessons) Warning: We are also told that we need to obey the gospel in 2 Thessalonians 1:7-9 (KJV)."And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with His mighty angels, in flaming fire taking vengeance on them who do not know God, and that obey not thegospel 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."
"For I am not ashamed of the gospel of Christ; for it is the power of God unto salvation to every one who believeth; to the Jew first and also to the Greek." Romans 1:16 (KJV)
"For God so loved the world, that He gave His only begotten Son, that whosoever believeth in Him should not perish, but have everlasting life." John 3:16 (KJV)
Although we need to believe in God's son Jesus(John 8:24), we also have to obey the gospel (2 Thessalonians 1:7-9). The devil believes Jesus is God's son, but that alone does not save him, because he doesn't obey God and has not obeyed the gospel. Read James 2:19 and Revelation 20:10. But how do we obey the gospel?