This is from the blog Things Paul & Luke:
It was during these days (cf. Mark 3:6-14; Luke 6:11-13 – i.e. the Feast of Tabernacles), that Jesus withdrew to meet with his Father in solitude on a mountain. After praying all night, he chose his twelve apostles from the disciples following him (Luke 6:12-16). It was at this time that he taught them what is summarized in the Sermon on the Mount (Matthew chapters 5, 6 & 7). After coming down the mount he met a multitude and healed them of all their diseases. In doing so, Jesus began to repeat key phrases of the sermon here on the plane, teaching the disciples through his works what he had told them on the mountain (Luke 6:17-49). 
Jesus’ teaching is one of contrast between our life in Adam and our life in Christ. Consider all we can know in this world without Christ, whether religion, politics, military, sports, music, commerce, education, science, arts, government etc. All these make up Adam’s world system. This world’s system is not of our Father in heaven (1 John 2:16). All these things were begun by the life of Adam, to satisfy the lust of Adam’s life, and are held in existence today by Adam’s life as it exists within all the people of the world (contrast Colossians 1:16-17).
The unbeliever lives out his life in Adam, while the Christian abides in Christ. These two entities cannot be more opposed. The one lives only to satisfy self, while the other lives to satisfy God within. In Adam, self is supreme. One may sacrifice for others, but the motivation is to satisfy one’s own conscience, personal pride, love for country or organization (religious or business) etc. The Christian lives as though he has given up all rights to these things and lives only to satisfy Christ within (Luke 9:24-26; Philippians 3:7-8). It is all a matter of supremacy. There is a daily struggle in the Christian’s life (cf. Galatians 5:16-25), because there is no agreement between the two lives.
The world looks for men with experience, people with confidence in themselves, men with drive and the ability to push other men forward (Luke 6:24-26). Nevertheless, it is not the way of Christ to drive or push others. Rather, he seeks men, who are willing to place themselves under the influence of his Spirit (Philippians 2:13), men who are willing to lead (not drive or push) others to him (Luke 6:20-23; cf. John 15:4-5). Men need to trust Jesus (John 14:1) and be willing to forsake what they could have had by living for the goals of this world (cf. Ephesians 4:22; Colossians 3:9).
The world’s idea of success is to have the things of this world in abundance (Luke 6:24), but the Christian’s abundance is Christ (Ephesians 1:3). The successful in the world live in fear of want (cf. Luke 6:25a) and go to great lengths to keep what they have and to accumulate more. The Christian gives Christ away and finds his abundance (Christ) has grown from within (cf. John 3:30). The more he releases and gives away, the more he has, and he has no fear of loss.
The religious person, whose certification lies in self, covets the good will of others (Luke 6:26). He will defend tradition based upon falsehood, because this is what the majority believes and treasures. His whole life is lived to satisfy this inner need to be congratulated, and anyone who challenges this self-worship is cast out of his company (Matthew 6:1-2, 5, 16). The Christian will lift up Jesus and will confront and admonish those who, while claiming the name of Christ, lift up someone or something else as their authority. In doing so he will make enemies of those who are carnally worshiping Christ, and he risks being cast from their company and held up to scorn in the eyes of his friends and brothers in Christ (Luke 6:22).
The lives of Adam and Christ are opposed to one another (Galatians 5:16-25). The one wants to live as the world lives, a way that feels comfortable, but the Christian’s Way is Christ (John 14:6). The unbeliever is always concerned over what belongs to himself, but the Christian finds joy in the fact that his life is not his own (1 Corinthians 6:19; Philippians 1:21). Jesus makes all the difference.
 Some folks claim Luke 6 and Matthew 5-7 are one sermon, but I find no difficulty in seeing two. After all, most pastors today often deliver at least two sermons every Sunday. Why is it so difficult to see Jesus doing something similar?
 I received this definition of the world from reading Watchman Nee – Love Not the World, chapter one, ‘The Mind Behind the System’ page 16.