Romans 3:27

Where, then, is boasting? It is excluded. On what principle? On that of observing the law? No, but on that of faith.

(v27, NIV)

All of a sudden the topic of boasting is brought up by Paul. Why is boasting the new topic at hand? Well, Paul has lengthened out about God and the gospel and Jesus Christ in the last section. There isn't much in there praiseworthy of man aside from what God offers to them. What Paul does say about people is that all have sinned and fall short of God's glory. If all this is about God and what he does, it's not really about us. We trade away his glory and choose images instead of God. So there doesn't seem much room for boasting. What becomes of it is that it is excluded. There is simply no room for boasting. If boasting is excluded, this begs the question as to how or why it is excluded. One suggestion is that observance of the law excludes boasting. Observance of the law is clearly not the way boasting is excluded. Instead, it is on the basis of faith that boasting is excluded. The observance of the law doesn't exclude boasting but it encourages boasting. When we observe the law, we are tempted to boast about our deeds. It is not on the basis of the observance of the law that boasting is excluded.