You must endeavour to know the commands of the law; not that you may be enabled, by that knowledge, to practise them immediately, and so to procure salvation by your works; but rather, by your knowledge of them, you may be made sensible of your inability to perform them, and the wrath that you are under for breaking them, and the impossibility of being saved by your own works; that so you may fly to Christ for refuge, and trust only to the free grace of God for justification, and strength to fulfil the law acceptably, through Christ.
While I'm on the subject of Walter Marshall's book, I would highly recommend it for anyone interested in justification or sanctification (and that should describe every Christian. . . ). It is the best book, bar none, I have read on the subject. In fact, it is quite possibly the best work of non-fiction I have read to date, excepting the Bible. The Puritans were master students of scripture, and Walter Marshall is no exception.