To have that all encompassing assurance you don't have to do a thing to earn it or even buy it. You just have to accept. Accept that Jesus is the Son of God and that He died and rose again and is seated at the right hand of the Father. So what exactly has that to do with inner peace?
To start, you have to understand that you were born with what is called a sin nature. There isn't anything your parents could have done to not have you born that way. It all started way back with Adam and Eve. They thought they could be like God and went against His instruction to not eat from the tree of knowledge of good and evil. One would think that it would be a good thing to understand the difference between good and evil but it didn't really work that way. To know good was to know God. To learn about evil caused an untold number of death symptoms to manifest in their bodies.
In other words they experienced a sin nature. Sin to most people is adultery, lying, fornication, stealing, etc. Yes, those are all elements of a sin nature. But did you know that sickness and disease are also sin?
Because Jesus died to take our sin upon Himself, removing the nature we were naturally born with, to carry it is the very essence of sinning. What? My cancer is a sin? Yes, but not what you did but what the enemy is trying to convince you that you have. I like to refer to sickness and disease as lying symptoms.
What Jesus died for is our release from anything that will cause death before our appointed time. To allow anything that causes death to reside in your body, be it adultery or cancer, is to say that Jesus died in vain. Adultery may only cause emotional death to a relationship but cancer can cause physical death to your body.
Accept Jesus into your life and live a stress free, sickness free life. Will trials and symptoms come? Yes, but how you believe can turn even the worst of times or bad reports around for the better.
Believe that Jesus can do what He did in the Word of God and what He says we can do because after you accept Him, you are also a child of God and an heir to all He did and said. Heirs always inherit the benefits in the will of the one who has died before them. The New Testament is the last will and testament from Jesus given to us for our inheritance.
Let me know if you have further questions. I may not know all the answers but I know the One who has them.