What is the main theme of the book of Galatians?

The book of Galatians primarily addresses the theme of recovering the true gospel by refuting the false teaching that salvation requires adherence to the law alongside faith in Christ. Paul expresses his astonishment at the Galatians' quick turn to a perverted gospel, where the danger lies in combining faith in Christ with human works, a concept exemplified by the 'Christ plus' mentality. This heresy leads people away from the pure grace of God, as Paul states in Galatians 1:6-7, asserting that there is only one true gospel. The urgency of this message is underscored by the doctrinal importance of maintaining salvation through grace alone, highlighting the necessity of faith in Christ without any additions.