Justification before God is by grace alone through faith in Christ alone. Any requirement beyond the exercise of faith reduces the Christian gospel to the level of the world’s false religions, making salvation to whatever degree a work of man rather than a work of God.
In Galatians, Paul argues that to import even the smallest human work into salvation is to be severed from Christ and to desert God. Christ-plus-anything is a recipe for spiritual disaster and eternal loss.
That makes Galatians one of the most important letters ever written. It is literally a life-saver.