From Anstey’s Doctrinal Definitions:
This refers to the Christian’s conviction to keep himself clear of the world—both in a religious and a secular sense. The following things are some reasons why God insists on separation in a Christian’s life:
Firstly, separation is necessary because association with the world will result in the restricting of the believer’s affection for Christ and His people. Simply put, it brings coldness into the soul. In 2 Corinthians 6:12-18, the Apostle Paul explains that our “bowels” (affections) will become “straitened” (restricted) if we neglect to walk in separation from the world. This sad effect is seen in the case of Ephraim who “mixeth himself with the peoples” of the nations (Hosea 7:8). The result was that he became “like a silly dove without heart” (Hos. 7:11). His association with the world took his heart away from the Lord.
Secondly, association with the world spoils the believer’s appetite for the Word of God. We see this illustrated in the history of Israel in the wilderness. God gave them “manna” for their food (Ex. 16). The New Testament tells us that this is a type of Christ who is the spiritual food for the believer (John 6:31-58). However, there came a time in their journey through the wilderness when they became tired of the manna, and this was because they were lusting after the foods of Egypt—a type of the world (Num. 11:4-6). Thus, believers who hanker after worldly pleasures and entertainments will have their appetite for the Word of God spoiled.
Thirdly, association with the world desensitizes the believer’s moral standards. The Christian who associates with worldly people will be influenced by them. He will begin to think and act like them, and their worldly values and moral standards will become his. The Word of God says, “Be not deceived: evil communications corrupt good manners” (1 Cor. 15:33). Lot’s moral standards dropped down to the level of the men of Sodom (Gen. 19). This is seen in the alarming fact that he offered his daughters to the men of the city.
Fourthly, association with the world hinders the believer’s spiritual growth. Again, this is seen in the case of Ephraim who mixed himself among the Gentiles. The result was that he was like a “cake not turned ” (Hos. 7:8). That is, he was like a half-cooked pancake—being only half developed.
Fifthly, association with the world causes the Christian to lose his spiritual energy in his personal life. Joshua warned the people in his day that if they settled down among the godless Canaanites, they would lose their power to resist them (Josh. 23:12-13). They would no longer be able to stand before their enemies. Again, with Ephraim it says, “Strangers have devoured his strength” (Hos. 7:9). Samson is another illustration of this. By his association with the worldly Philistine girl (Delilah), “His strength went from him” (Jud. 16:19).
Sixthly, association with the world will cause the believer to lose his spiritual discernment. Again, the example of Ephraim illustrates this. It says, “Strangers have devoured his strength, and he knoweth it not: yea, gray hairs are here and there upon him, yet he knoweth not” (Hos. 7:9). Having associated with the world, he became insensible to his personal state. There was spiritual decay, and he didn’t know it! It was the same with Samson. He didn’t seem to know that he had lost it either. It says, “And he wist not that the LORD was departed from him” (Jud. 16:20). The Laodiceans were also devoid of their true state and imagined that they were right and good, but their state was actually obnoxious to the Lord (Rev. 3:14-22).
Seventhly, association with the world will eventually turn a believer away from following the Lord altogether! The children of Israel are an example. They were not to mix with the nations in the land of Canaan who knew not the Lord because those people would turn them away from the Lord (Deut. 7:1-4). They didn’t heed this warning and that is exactly what happened
Lastly, association with the world destroys the believer’s personal testimony. Lot is an example. By living in Sodom (a type of this world in its moral corruption) he lost his power in testimony. When he went to call his sons-in-law out of that city which was under judgment, his words to them seemed “as one that mocked” (Gen. 19:14). They didn’t take him seriously because his life told another story. Christians generally have not kept separate from the world, and as a result a poor testimony has been rendered before the world. It’s a wonder that anyone turns to the Lord and gets saved. Gandhi, of India, said that if it weren’t for Christians he would have become one!