"The peace of Christ must control your hearts - a peace into which you were called in one body" (Col 3:15, CEB) "Come unto me, all ye that labour and are heavy laden, and I will give you rest (peace)" (Matt 11:28, emphasis mine) Salvation is simply a call to come have some peace. There are lots of things out there troubling us, stealing our peace, disturbing our lives. And Jesus comes around to restore peace. Peace was the first thing sin took away from man. As soon as Adam and Eve sinned, they became at war with themselves. They lost their composure, their confidence, their calmness - they became fearful. When they heard the voice of God, the very same voice they have been hearing everyday, they went and hid themselves. A heart at war with itself sees troubles where none exists. It runs when no one is in pursuit of it. It dies a thousand times before death even comes..
The reason God wants us to be at peace with ourselves is because life is terrible and miserable otherwise. If you don't have self peace, you will see ghosts where none exists. And the origin of all self discord is sin. This is why the Bible says that we have been called into peace. Our salvation guarantees us peace. It sets us free from the yoke of the evil one; from slavery to the flesh and its desires; and gives us the new life of the Spirit. That new life is characterized by peace. For the fruit of the Spirit is love, joy, peace etc (Gal 5:22)
Peace isn't just calmness; it is the ability to stay calm even in the midst of troubled waters. Peace is not external; it is internal. It does not depend on what is going on around you, it depends on what is going on inside you. If your heart is at peace, if the Holy Spirit - the Spirit of peace dwells in you, you will have peace even if you walk through the valleys of the shadow of death. Your peace does not come from you, it comes from God. Peace with God is peace with self.