In Romans 3:27 Paul contrasts the 'law of works' with 'the law of faith'.
Since works are not required by the law of faith (because it is of faith) therefore works are excluded. For if boasting is excluded, as Paul insists it is, then works must also be excluded.
For faith is not works. And works are not faith.
Yes, it is true that good works will, indeed, flow from faith if the faith be genuine. But they will be 'the work of faith' and 'the labour of love', 1 Thessalonians 1:3. They will not be legal works aimed at self-justification. For by faith, one is already justified and if that faith is the 'Abraham' kind of faith, then a witness will be received (as Abel received, Hebrews 11:4) that one is, in God's sight, 'just' or 'justified'.
Then what rule shall one walk by ?
Paul answers the question in Romans 8: 1 and 2.
There being, now, no condemnation to them that are in Christ Jesus (for they are dead with Christ who are 'in' Christ Jesus and they are no longer under the law, that being dead - the old humanity - wherein they were, in times past, held, Romans 7:6) who walk not after the flesh (for if we live in the flesh we shall die, Romans 8:13, for the law will speak to us and sin will rise up, see Romans 7, and will slaughter us, all over again, as we know by bitter experience) but after the Spirit.
Walking thus, after the Spirit and (as Paul diligently makes very clear in the Galatian epistle, Galatians 2:19 for example, and in Romans, 7:1-6, for example ) freed from the law (for the flesh is dead, in Christ, Romans 8:6 and 10) we need a rule to walk by.
And there it is, Romans 8:2 'the law of the Spirit'.
And the rule of the Spirit is that 'life is in Christ Jesus'.
Running the words together as 'the spirit of life in Christ Jesus' is ungrammatical. If the meaning was that the 'life in Christ Jesus' had a spirit, the wording would not be as Paul dictated to Tertius to write down on the page. It is νομος του πνευματος της ζωης εν χριστω ιησου.
There is an article before 'life' : it is 'of the life' or (since the Greek article derives from the demonstrative pronoun, this/that, see Daniel B Wallace) one could say 'rule of the spirit of that life in Christ Jesus'. The rule of the Spirit relates to that life which is in Christ Jesus.
The rule of the Spirit - of the life in Christ Jesus - is the same rule as the rule of faith.
Faith excludes legal works.
And so does the rule of the Spirit.
For life is not by law. We learned that (or we should have learned it) in Eden. It brings death : inevitably. Because (prior to a new birth) we are creatures.
Life is in Christ Jesus.
And this is a 'rule' : it is, indeed, a 'law' just as the 'law' of gravity is a 'law'. It is a rule. It is invariable. It always works. It is never otherwise. The word νομος, nomos, is a broad concept in Greek and is used in that sense in the bible. It does not (as some seem to want it to do so) invariably and universally mean 'the law of Moses'.
But this is not a new 'law' to burden us and to bring us back under bondage. It is a rule to walk by : that life is in Christ Jesus.
'Take my yoke upon you and learn of me' saith Jesus ' for my yoke is easy and my burden is light', Matthew 11:29.
But this old nature that we still carry with us, like a rotting carcass, will, ever and anon, lead us back to the tree of the knowledge of good and evil, which is where we all went wrong in the first place, in Adam, in the beginning of humanity.
But the baptism of repentance, as ministered by John the Baptist, the Messenger of Preparation, see Malachi 3:1, will fill us up, like six waterpots of stone, until, filled, the water turns to wine, John 2:1-11.
This birth is not only a 'birth again' (anagenesis, 1 Peter 1:23) and a 'birth from above' (anothen, John 3:3) it is also a 'reversal birth' (paligenesis, Titus 3:5).
The new birth reverses the old birth and goes clean against the old nature and its inclinations to self-righteousness and independence of the Spirit. And this new birth leads us to the rule of the Spirit.
Which is : that Life is not in the tree of knowledge (of good and evil).
But Life is in Christ Jesus - the tree of life in the midst of the garden.
There is no life in law. It has none to give. It only has a commandment, 'This do - and thou shalt live', Luke 10:28. But I have no life to 'do' . . . .
. . . . for Life . . . . . is in Christ Jesus.
'In him was Life, and the Life was the Light of men.' And if we walk by the rule of the Spirit, then we shall we walk in the Life that is of Christ (as raised from the dead, in Him, Ephesians 2:5) and we shall see where we are going for 'in thy light shall we see light', Psalm 36:9 . . . .
. . . . and we shall not stumble.