No one can have the peace of God until they are first at peace with God and those who are born again should have this peace (Rom 5:1) because they’re not under condemnation anymore (Rom 8:1). Whoever has their mind “stayed" on God, He promises to keep them “in perfect peace." Why you might ask? It’s because they trust in Him. Trust or faith is the other end of the spectrum of doubt and fear. It’s so easy to have times of sadness when we take our minds off of the great and precious promises of God and “he has given us great and precious promises. These are the promises that enable you to share his divine nature and escape the world’s corruption caused by human desires" (2 Pet 1:4) and every one of those promises are found as sure in God (2 Cor 1:20).