Habakkuk 3:2 (ESV)

2 O LORD, I have heard the report of you, and your work, O LORD, do I fear. In the midst of the years revive it; in the midst of the years make it known; in wrath remember mercy.

The prophet addressed God with the words, O Lord to acknowledge his power and display his humility before him. The report he heard may have referred to what God said he would do to his people in Habakkuk 1:5, Look among the nations, and see; wonder and be astounded. For I am doing a work in your days that you would not believe if told. God was planning to raise up the Babylonians to inflict judgment upon his people. This caused Habakkuk to fear, meaning that he was awestruck by the announcement of what he was going to do to them. His response of fear showed that he believed God’s message to be true which expressed his faith, trust, and confidence in him.

The words, in the midst of the years referred to the time between God using the Babylonians to chasten the Israelites and in turn punishing the Babylonians. During this period, he asked God to revive the righteous ones who lived their lives by faith (Habakkuk 2:4). He requested that he show his mercy to them, to be tender and compassionate toward them. Nothing but the undeserved mercy of God would sustain the Israelites during the affliction that the Babylonians would inflict upon them.