Habakkuk is quite a difficult book for us to get into, after all when was the last time you sat and read poetry. It is a book designed to create in us an emotional response. I think sometimes we are very afraid of our emotions and are in danger if divorcing our emotions from our worship or prayer. But Habakkuk shows us that robust faith responds emotionally to distress and injustice whilst maintaining a robust faith in the character of God.
One of the issues that was raised in the discussion afterwards was about application. Is the application of the first section to unbelieving persecution of believers, or persecution from other believers, or those who would call themselves believers? I think in Habakkuk's time the persecution was primarily led by the king, and it was oppression of the remnant faithful to God among a generation that was unfaithful to God and disbelieving. In short I think the application is both; we ought to mourn as we see believers oppressed and their fundamental rights denied and justice twisted into injustice by unbelieving society. But in some ways we want to mourn all the more when such things come from those who claim to share their faith.
In Britain this is increasingly happening within denominational circles; as those who stand on the word of God are attacked by those who do not. It is also happening in the law courts; as laws which were based on a Christianised past are repealed or changed, or ruled on in a way which hems in believers.
How ought we to respond? I think Habakkuk calls us to mourn but to turn to the sovereign God who is our deliverer and who keeps his covenant.