Romans 5:1 — Therefore being justified by faith, we have peace with God through our Lord Jesus Christ.
The theme of chapter 5 is "through Christ."
Therefore — The subject is linked back to chapter 4.
Being = having been
Benefit of justification — peace with God (not "peace of God"). It's a state, not a feeling. Sinners are at war with God, the justfied are at peace.
Jesus Christ is our mediator and our peace.