By faith

knowing that a man is not justified by the works of the law but by faith in Jesus Christ, even we have believed in Christ Jesus, that we might be justified by faith in Christ and not by the works of the law; for by the works of the law no flesh shall be justified. - Galatians 2.16

The judaising Galatianists could not get away from the necessity of the Law. They said that even if you were saved you were not really justified unless you kept the Law. 

The problem with justication is that to be justified to God's standard one must be perfect and the problem with perfection is that no one can achieve it. No matter how good the works of the Law are they will always fall short. James tells us that to neglect one point of the Law is the break the whole Law. 

Paul is laying the foundation here for a response to the judaisers. If we cannot be saved by works we can't be justified by works. Therefore works cannot be part of what is required of us. It is faith in Christ that saved us and faith in Christ that motivates us to good works. 

We can't let the grace robbers steal our joy in Christ by adding the burden of 'do this don't do that' to our Christian living.