Justification highlights how our salvation is God's work, not our work. It is not that we don't do anything and just wait around for God to lift us up out of this world into heaven. No, we do have a lot to do, especially in our sanctification. Even in justification, we must believe. But none of what we do actually counts toward our salvation: "all our righteous deeds are like a polluted garment…" (Isa. 64.6). If what we do as Christians is acceptable to God, it is only because Christ's righteousness covers the deficiency of our actions.
How, then, are we justified in Christ? In Rom. 4:5 we read, "And to the one who does not work but believes in him who justifies the ungodly, his faith is counted as righteousness…." This is a wonderful statement, affirming the amazing grace by which sinners like us are saved. But to know how truly wonderful this statement is, we need to first understand how troubling this statement is from a legal perspective. Here, God is described as the One "who justifies the ungodly…." If we take our gospel lens off and read this statement, we should be horrified. To justify in the legal setting is to declare someone innocent because he is innocent (cf., Deut. 25:1, NASB).