The 'gospel' isn't just 'good news'; it's the specific, unchangeable good news about Jesus Christ. Any alteration makes it a different, dangerous message.
What is the True Gospel?
The gospel is the death, burial, and resurrection of Jesus Christ for the forgiveness of sins, enabling reconciliation with God. It's the core of Christian faith, centered entirely on Jesus' finished work on the cross.
The Peril of a Different Gospel
A 'different gospel' might add human effort, downplay sin, or change the object of faith. It might sound appealing or inclusive, but it cannot save. Paul calls such messages 'another gospel' which is 'no gospel at all' (Galatians 1:7). It offers false hope and leads people away from the saving power of God through Christ alone.
The Corinthians' Complacency
Paul's exasperation ('you put up with it readily enough') highlights the danger. People were tolerating these deviations, perhaps because they were subtle, or because they valued peace over truth, or because they had lost their biblical discernment.