I John 2:15-17 warns us that there is a profound gulf between the Father and the world, and that a Christian is faced with making a choice between them. Spiritually, morally, and ethically, Christianity does not allow for neutrality. God is bringing us into a position where we recognize truth, admit it is true, and make it a part of our lives.
We are learning a new way of life, so He does not want us to be ensnared by the attractiveness of many things that are in the world. We cannot presume that because something appears to be harmless, it would be fine to do "just this one time." Therefore, we have to learn to resist the urge to think and conduct our lives as the world does.
"World" in I John 2 is the Greek cosmos, and its basic meaning is "an ordered system." Because of the disparity between God and this world, it cannot possibly be the world for which God gave His only begotten Son. The world He created He called "very good." Nor is He referring to mankind, also part of His creation. He loves people and desires to save them.
Nevertheless, He does not like man's way of life. This ordered, human-centered system is anti-God and anti-Christ, and Satan sits at its head. This system occupies His creation and consists of people that God loves so much that He sent His Son to die for them, but He does not love the system! It produces people that need to be rescued, and it tends to make them worse and worse.
When God speaks of "the world," He is identifying all of man's purposes, pursuits, pleasures, practices, and places where God is not wanted. Much of this world is religious, cultured, refined, and intellectual, but it is still anti-God and anti-Christ.
Through His calling, God puts us into a position where He forces us to choose between disparate ways of life, and both of them are realities. We must choose either the eternal and worthwhile or the temporal and vain. God is not saying that this world is unpleasant, unattractive, or unappealing, but we have to choose between that reality and His. The sum of this passage is that this ordered system—anti-God yet appealing and attractive—has the power to seduce the believer, to ensnare him and turn him from God. We have to be vigilantly on guard against it.