。
It's our duty to obey the law.
Literal
Law [dat-に] obey [nominalizer-の] [topic-は] our [possessive-の] duty-[assertion-だ]
従う ('to obey, follow') takes the obeyed party in に, not を. Nominalizing 法律に従う with の lets it be topicalized and equated with '我々の義務'. A textbook pattern for 'X is Y' where X is a whole clause.