Chapter 35 - Third Use of the Law
Some Christians are allergic to the law. Not under law, but under grace is their motto, and if someone points them to a moral obligation in Scripture, they'll likely look squinty-eyed, suspecting legalism. They have forgotten that Christ freed us to serve and obey the Lord (Romans 8:4).
Other Christians live by the law. If they are going to make any progress in godliness they have to get serious about obedience. Or they have to study Scripture harder to understand the law. They have forgotten that the Spirit of liberty lives in them (2 Corinthians 3:17).
The Puritans emphasized something called the third use of the law, which strikes the biblical balance between these two unhealthy tendencies. We don't grow in godliness through condemnation or through casting off all restraints including biblical ones, but by looking to the Word for guidance to live.
The law will not justify us. It is not the source of our life in Christ. But the Spirit gave it to us to help us follow Christ, AFTER we come to Him to forgive our sins. Luther said that outside of Christ the law is a beating stick, but after conversion it is a walking stick for us to walk in godliness.
Westminster treats the Ten Commandments in a way that draws out its application to us in this third use. Some evangelical Christians lean anti-nomian (literally, against law - described in the second paragraph, above). They are concerned by the language of duties and obligations, but Jesus didn't come to abolish the law, and gave plenty of stern warnings and commands Himself based on it.
None of this means we are saved by the law, or should rely on our obedience or works to get favor with God. But Gospel and Law are more friends than they are enemies, to the true Christian, once his condemnation is lifted. Being truly forgiven motivates you to love and serve. It is an abuse of forgiveness to go on sinning because you don't care and can get away with it.
I'm not quite sure how this chapter fits in the Soteriology secion. The Puritans tended to cover their favorite topics (like this one) in every section of theology, and their followers, like Beeke/Jones, tend to do the same.