In John 15:19, Jesus told His disciples that they were not of the world and that He chose them out of the world. In other words, Jesus told His disciples that even though they were living in the physical world, they were not to have the mental attitude or value system that the worldly system has.
Since Jesus saw how much the worldly value system rebelled against God’s value system ever since the original rebellion against God’s value system by Adam and Eve; then Jesus wanted His disciples to come away from that system so that they could have a value system based on His teachings and God’s principles of living found in the Bible.
The foundation for developing a Christian value system is to believe that God is real and has revealed Himself, His actions, and His principles for living in the Bible. Since the first chapter of the first book of the Bible says that God created mankind (Genesis 1:27), then God has the right to tell mankind the laws and principles that He wants mankind to live by, as well as the right to impose punishment for whoever breaks those laws.
When God’s original creations, Adam and Eve, were seduced by the serpent/devil into breaking God’s command not to eat the forbidden fruit of the tree of the knowledge of good and evil (Genesis 3:1-6); then the holy nature of God had to impose the spiritual sentence death that He warned them about. (Genesis 3:3)
However, at the same time, God’s great love for His special creations, (and the awareness that they would pass their sinful nature and the tendency to rebel against God’s laws down to their children and succeeding generations) caused God to develop a plan for the spiritual redemption of mankind that would eventually overcome most of the problems that the rebellion of Adam and Eve caused to the relationship between God and mankind.
This redemptive plan involved sending His Son, Jesus Christ, to come to earth, become born as a man, live a sinless Jewish life according to God’s commands, teach people how God wanted them to live, and then show God’s great love for mankind by becoming the perfect Passover sacrifice as the Lamb of God when He died on the cross fulfilling God’s plan for the spiritual redemption of mankind.
Since Jesus lived a perfect life and didn’t break any of God’s laws, then as the perfect Passover sacrifice He could take the punishment of death that all people who broke God’s laws deserved. (Hebrews 10:10-14)
As the result of Jesus completing God’s redemptive plan, then everyone who accepts God’s invitation to ask God to forgive them for breaking His laws and then asks Jesus to become their personal Savior and spiritually apply His sacrificial atonement on the cross to their lives will be forgiven by God for breaking His laws, made God’s newly restored creation, and be given God’s Holy Spirit to live inside of them to give them the insight and power to live according to God’s principles.
Then, as people daily study God’s Word to learn those principles and talk to God in prayer, they can develop a close relationship with God that will not only help them with their spiritual life; but it will also help them with their physical life by giving them a godly and Christian value system to live by.
The more Christians live by God’s principles and use their Christian value system to make important decisions, the more they will spread the light of God’s kingdom to show other people the way to come into God’s kingdom. Then, after they have successfully completed their journey of life, they can look forward to their reward of living with Jesus in His Kingdom of Heaven forever.