Rejoice in the Lord alway: and again I say, Rejoice.  Let your moderation be known unto all men. The Lord is at hand.  Be careful for nothing; but in every thing by prayer and supplication with thanksgiving let your requests be made known unto God.  And the peace of God, which passeth all understanding, shall keep your hearts and minds through Christ Jesus.
Philippians 4:4-7

To have the peace of God, you need to be at peace with God (Romans 5:1-2).