Paul is applying a maxim: where there is no law, there can be no violation of law, hence no penalty. Law is a standard. Law can't confer any blessing, there is no reward for merely keeping law, or doing one's duty. However, the law does impose wrath, through conviction--falling short of the standard, failing to measure up. Law reveals perfection or imperfection. Law can never solve the problem of lawbreaking. "That is why it depends on faith..." (v.16).