There is only one true gospel, the gospel of the Kingdom of God that Jesus preached, and there are many false gospels. Perverting it began early, when preachers shifted from what Christ preached to the person of Christ. Paul warns that a different gospel is not even good news, since it ends in death rather than eternal life. False gospels mix a little truth with error, like poison in water, so the poison remains and the whole is false. Those who preach them are accursed. John identifies the spirit of antichrist as any teaching that changes the nature of God and Christ, such as the Docetic claim that Jesus and the Christ were two beings, and he gives tests: whoever denies that Jesus Christ came in the flesh is not of God, and the world hears false teachers because they sound worldly. Christ taught that false teaching is known by its fruits. True teaching produces character and eternal life, while false teaching produces destruction and death and remakes people in the image of a god of their own making. Paul adds further tests. A false preacher seeks to please men rather than God, and his teaching originates in the human mind rather than in revelation from God through His Spirit and Word. There is no middle ground between Christ's side and Satan's, and wholehearted love for God, offering oneself as a living sacrifice, guards against error. Those who reject the true messenger reject the message.

Within only two decades of Christ's death and resurrection, someone had perverted the gospel into a different message that was no longer good news (Galatians 1:6-7). The true gospel is not of human origin; it is revealed by God through supernatural disclosure, so no one finds it simply by picking up the Bible, and God must open the mind to receive it because the carnal mind is enmity against God (Galatians 1:11-12; I Corinthians 2:7-16; Romans 8:7). In the first century the apostles fought two false gospels. Legalism, rooted in Judaism, taught that salvation comes through works of the law rather than grace. Paul countered that salvation is by grace, though good works are a necessary part of Christian growth and are what God is working to accomplish in believers (Galatians 5:1-6; Ephesians 2:8-10). Gnosticism, a group of heresies, taught that knowledge (gnosis) brings salvation and that spirit is good while flesh is evil. It devolved into asceticism, hedonism, and strange ideas about God and Christ, and it eventually came to dominate Christianity, with modern descendants proclaimed every Sunday. The most pervasive false gospel today is the gospel about Christ, which preaches the Messenger rather than the Message He brought. Christ's life is an example to study, yet when He preached, He revealed the way to the Kingdom of God. He commissioned His disciples to proclaim the Kingdom, never to preach Himself, and He spoke of it even after His resurrection (Matthew 10:7-8; Acts 1:3). The true gospel is the revelation of the imminent Kingdom of God, and any other gospel, even from an angel, is accursed (Galatians 1:8).

Any alteration or shift of focus from Jesus' original announcement turns the gospel from good news into bad. Jesus came preaching "the gospel of the kingdom of God," calling people to repent and believe it (Mark 1:14-15). Within two decades of Christ's death, Paul rebuked the Galatians for turning to "a different gospel, which is not another," a perversion of the gospel of Christ (Galatians 1:6-7). A changed gospel changes its goal, so those who follow it will arrive somewhere other than the Kingdom of God. The Bible is the source of Christ's gospel for the whole Christian world, yet the gospel preached by the church of God differs from Protestant and Catholic versions for one reason: it believes the gospel Jesus preached. That means believing time is short, that Christ will soon establish the Kingdom, that repentance is a way of life, and that Christ's message must be lived in faith. Claims of faith are tested by Jesus' words, "You will know them by their fruits" (Matthew 7:16). A false gospel shows in whether people try to live by every word of God, tremble before it, keep all the commandments, repent and overcome failings, grow steadily in character, focus on the Kingdom, put God first, center their teaching on Christ's proclaimed message, and show genuine love for the brethren. Since all stumble in many things (James 3:2), no person or group will likely pass or fail every test. Still, judging righteously whom to fellowship with requires a thorough grasp of the true gospel.

Gnostic gospels such as the Gospel of Judas and the Gospel of Thomas are false gospels. They appeared centuries after Matthew, Mark, Luke, and John, and they contradict the Bible. Gnosticism, the "falsely called knowledge" (gnosis) that Paul warns Timothy against in I Timothy 6:20-21, was the chief heresy of New Testament times, and John, I Corinthians, Galatians, Colossians, I and II Timothy, Jude, and I John all combat it. These false gospels open by stressing secret knowledge. The Gospel of Judas makes Judas the hero, who alone received secret revelation. The Gospel of Thomas promises that whoever discovers the interpretation of its hidden sayings will not taste death, so eternal life comes through knowledge rather than a relationship with God or a godly life. They also contain statements that oppose Scripture, such as the claim that fasting, prayer, and giving bring sin, condemnation, and harm, and that a woman must become male to enter the Kingdom. Their teaching rests on a dualism that treats spirit as good and flesh as evil, which contradicts Genesis 1:31, and on the idea that a divine spark is trapped in the body and freed by knowledge. Many Gnostics also held that Christ entered the man Jesus at baptism and left before the crucifixion. This heresy persists today in the immortality of the soul, in the feminist and liberal church use of texts like the Gospel of Mary and the Pistis Sophia, and in the rejection of God-ordained leadership. It also appears in popular works like The Da Vinci Code, which proposes repressed secret knowledge about Jesus. God establishes doctrine and leadership through those He chooses.

Churches disagree about what the true gospel is, calling it the gospel of grace, salvation, Christ, God, or the Kingdom, yet it matters greatly which is believed, since Paul pronounces a double curse on anyone who preaches a gospel different from the one the apostles preached (Galatians 1:8-9). The only true gospel is the one the Bible presents, and it is the gospel of the Kingdom of God that Jesus Himself proclaimed (Mark 1:14). The gospel of God, the gospel of Jesus, and the gospel of the Kingdom are one and the same message, originating with the Father and delivered by the Son. Even the gospel of grace is part of it, since Paul's preaching of grace was a preaching of the Kingdom (Acts 20:24-25; 28:30-31). A false gospel results when people accept Jesus as Savior and coming King while rejecting the message He brought, which calls for repentance, turning from sin, and keeping God's commandments. Another distortion is the mistranslation of Luke 17:21, which leads people to think the Kingdom is merely an inner quality of the heart. Jesus was actually telling the Pharisees that the Kingdom's King was in their midst. The true gospel announces a literal Kingdom that Christ will establish on earth at His return, and it also teaches how to live God's way of life now in order to share in it.