“We who are Jews by birth and not ‘Gentile sinners’ know that a man is not justified by observing the law, but by faith in Jesus Christ. So we, too, have put our faith in Christ Jesus that we may be justified by faith in Christ and not by observing the law, because by observing the law no one will be justified.”
Paul establishes a contrast between Jews and Gentiles in proving his point about justification. Jews lived their life in an attempt to live according to the law. In the end they failed miserably because they were unable to follow the law in the slightest bit.
Paul warns Gentile believers who did not become Christians out of the Jewish system that they can’t earn their salvation. Even though we understand that it is faith that saves us, there is still a tendency to strive to follow the law in an effort to earn or maintain our salvation.
There are good reasons to follow the law – our love for God, our desire to avoid the consequences of sin, and our yearning for holiness – but these are unrelated to our salvation. Justification by faith means that we do nothing to earn our salvation and no sins can cause us to lose our salvation.
We are justified by faith and as long as we maintain that faith in Jesus, we are saved. Whether we can stop having faith and thus lose salvation is a different issue that won’t be discussed here. As far as following the law goes, it didn’t save us and it won’t keep our salvation. We are justified by faith alone.