I have heard that when the covered wagons headed west across the frontier, they would occasionally run into a prairie fire. One thing they could do was light another fire downwind from themselves. They would then move the wagons onto the spot that was burned over by the second fire. They would be safe from the first fire since they were on an area that had already been burned. In the same way, God has caused the punishment due our sin to burn out on Jesus Christ that we might go free (1 Peter 2:24,25).
But it is easy sometimes to fall into the mindset of “Surely there is something I need do to make up for my sins. Surely it is not enough just to trust Christ for my salvation (Ephesians 2:8,9). Surely it is not enough afterwards, when I fail, for me just to admit my sins to God (1 John 1:9). Surely I need to do something good to make up for what I’ve done. Surely I need to do some sort of penance. Surely I have to fast or pray or make a pilgrimage to Jerusalem. Shouldn’t I at least go around feeling guilty and lamenting my sin?” We have the feeling that something must be done to pay for our sin. We are correct. But we are totally incapable of making such a payment (Isaiah 64:6, Romans 3:9-20). This payment was made for us by Jesus Christ (Romans 3:21-31; Ephesians 1:7).
The whole point of the cross was that Jesus paid the full price for our sins (Galatians 2:21). The cry “It is finished” (John 19:30) can also be translated “paid in full” (see Colossians 2:13,14). We are justified before God (Romans 4:5, 6; Galatians 2:16, 17), which is the verdict of the Judge declaring us righteous (see Deuteronomy 25:1). Therefore, not only God’s love but His justice is enlisted on our behalf. We are not condemned because the price is already paid (Romans 8:33,34; John 3:18). This does not mean we live our lives in disobedience to God, but rather we live for God out of love of Him for what He has done for us (2 Corinthians 5:14-15; 1 John 4:19; Titus 2:11-14). But we do not live in the old attitude of fear (Romans 8:15; 1 John 4:17, 18; 2 Timothy 1:7), but in love of the One who said, “Take courage; your sins are forgiven” (Matthew 9:2).