The steadfast of mind You will keep in perfect peace, because he trusts in You. (Isaiah 26:3)
To have and enjoy the supernatural, perfect peace of God, you must first have peace with God. Peace with God comes by faith alone in the gospel of the Lord Jesus Christ; Therefore, having been justified by faith, we have peace with God through our Lord Jesus Christ. (Romans 5:1) But even Christians are sometimes anxious and lacking the supernatural, perfect peace of God. In Philippians Chapter 4, we are given God's requirements for having His peace, even in the most stressful and difficult circumstances.
Be anxious for nothing, but in everything by prayer and supplication with thanksgiving let your requests be made known to God. And the peace of God, which surpasses all comprehension, will guard your hearts and your minds in Christ Jesus. (Philippians 4:6, 7)
Finally, brethren, whatever is true, whatever is honorable, whatever is right, whatever is pure, whatever is lovely, whatever is of good repute, if there is any excellence and if anything worthy of praise, dwell on these things. (Philippians 4:8)
The things you have learned and received and heard and seen in me (the Apostle Paul), practice these things, and the God of peace will be with you. (Philippians 4:9)