Hebrews 12:14-15 (NKJV)
Pursue peace with all people, and holiness, without which no one will see the Lord: looking carefully lest anyone fall short of the grace of God; lest any root of bitterness springing up cause trouble, and by this many become defiled;
There is no better proof of a changed life that the ability to live in peace among everyone. The kind of peace spoken of here is not superficial nicety. We know this because it is linked with holiness. Such peace is a deep love founded on the basis of a redeemed nature. And we know this because it says you will not see the Lord with out it. It is the holiness of a pure life made possible by the new birth we have in Christ:
Hebrews 10:10 (NIV)
And by that will, we have been made holy through the sacrifice of the body of Jesus Christ once for all.
God works this holiness into us through his discipline which He gives "for our profit, that we may be partakers of His holiness." (Heb 12:10)
The opposite of this peace is bitterness, a danger which is all too near. The bibles says we should beware and not allow a root of bitterness spring up. The soil for this root is discontent, a feeling that we have been wronged. Bitterness is never satisfied; it only ends in despair and destruction and drags others down with it. But when Christ is your Lord and master, what is your rightful basis for feeling wronged or slighted? There is a miracle that occurs in the life of a Christian under attack; it happens when he is able to deflect these feelings onto Jesus.
I remember being personally and unfairly attacked in a business deal about ten years ago. This situation went on for several months and weighed heavily on my mind. I was tempted to become defensive and to lash back. I remember walking one evening while praying and coming to the place where I was able to look to Jesus for all my satisfaction. I remember very well how free I felt knowing that I was working as to the Lord and not to men. He would be my reward. I was able to resolve the situation without any trace of bitterness or sense of revenge. That situation had such an impact on my life that to this day I can clearly see the potential for bitterness long before it has a chance to take root. The resulting peace and holiness that come from such a victory can only be described as joy.
Hebrews 12:3 (NKJV)
For consider Him who endured such hostility from sinners against Himself, lest you become weary and discouraged in your souls.