There are two laws in view in this context. So which law justifies and which law does not? The faith which justifies in verse 27 is the law of faith mentioned in verse 26. Law is a rule of conduct. Many people who hold to the doctrine of justification by faith only often use this verse out of context as a proof text in support of their doctrine.
The law of faith goes by other names in scripture:
All of these terms point to a rule of conduct for the Christian to observe. The "deeds of the law" is therefore a reference to the Levitical Law and Paul is drawing a contrast between it and the law of faith under which we live now.
Romans 3:28 Paraphrase: