Jesus said:
“The Kingdom of God is near! Repent of your sins and believe the Gospel!” (Mark 1:15)
These words may be regarded as a summary of the method of salvation. Repentance and faith are the conditions for adoption into the new Christian covenant. Repentance is a special reference to God the Father and recognition of our offense against him. Faith is a special reference to our Savior Jesus Christ, our righteousness found, not in ourselves by repentance, but found in Him by his grace. Repentance of itself is not sufficient. Without faith repentance becomes despair, and without repentance faith becomes only presumption. Join the two together, and the faithful soul is born again.
Why repentance?
By repentance we give glory to our Creator whom we have offended by our sin.
Why believe (faith)?
Believe the gospel - Literally, trust in the gospel. Trust Jesus and follow him obeying his word. Jesus, the Son of God, Creator of all things, was begotten as a man to witness to the truth, and those who believe are born again and are saved, forgiven of their sin, they find friendship with God. By faith we give glory to our Redeemer who came to save us from our sins. His Gospel scheme; he proclaims liberty from the law, peace, pardon, and righteousness in him, and salvation and eternal life by the free grace of God the Father. Pardon from what? Christ Jesus purposely joins these two, redemption and sin, together so that no one would get away with trying to leave off the repentance. Those whom Christ calls, must leave all to follow him, especially their sin, forsaking every thing that is against their duty to Christ.
How's does this proclamation differ from that of John the Baptist? The word “believe”, it's a new word being joined to the call to repent.
Believe what?
The gospel.
What is the gospel?
In short, the gospel is the news that Jesus Christ, king of the universe, died for our sins and rose again, eternally triumphant over all his enemies, so that now there is no condemnation for those who believe, but only everlasting joy.
Even shorter, it’s the message about Jesus.
Overall it's the good news of the kingdom, and everything that Jesus tells us about His kingdom (See Matthew 13). And the apostolic methods for proclaiming the gospel, was focused on Christ: who He is, what He did, and how we receive His benefits. If your Pastor tells you that if you come into relationship with Jesus you can find a purpose, meaning for your life, or you can have peace in your soul, or God loves you and has a wonderful plan for your life, that all may be true, but that’s not the gospel. Don’t confuse that with the gospel. The gospel is about Jesus. The apostles didn't march into the synagogues and temples proclaiming your best life now.
The apostle Paul told us what the gospel is: "For l delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures..."
1 Corinthians 15:3–4
Every gospel message is Holy Week. Every gospel message is about Jesus and what HE did, as Paul says, "according to the scriptures".
Paul says in the letter to the Romans, twice, “I want to explain my gospel to you.” And there he makes the statement, “Christ died for our sins according to the Scriptures; and on the third day He rose again."
That's the gospel...the resurrection of Jesus Christ.
And your best life now? Resurrection in Him.
Relationship with what? Resurrection in Him.
Born again "In Him", in what? Resurrection in Him.
Receive the good news, repent, and believe.