Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ. (ESV)
I have heard many a pastor say, "When you see the word "therefore" in the Bible you need to pause and see what it is "there for." Paul has spent the first four chapters establishing that we are saved by faith, not by works. This is how we are justified, or declared righteous. The "therefore," will now tell us what the result of our saving faith is.
We have peace with God! At first glance it would be easy to understand this to mean that we have a sense of calm replacing the uneasiness or worry that used to be in our hearts. While it is true that the Lord is able to give us that kind of peace, Paul is talking about something completely different in this passage.
We have peace with God. That phrase means that the war is over. We were in rebellion to God before salvation. We actually were aligned with the "Prince of the power of the air," Satan. Because we have surrendered and given our allegiance to God we are no longer God's enemy. Rather, we are considered friends, heirs, even children.
Before you can have the peace "of" God you need to have peace "from" God. If you have placed your faith in the finished work of the Cross you no longer have to fear the wrath of God. That wrath was placed on Jesus. He took your punishment. You are now enlisted in a new army. You fight for the kingdom of God now.
Therefore. Faith in God puts up the white flag of surrender. The war is over. He wins. And in the process you win as well. Enemy to heir.