John writes: Do not love the world (1 John 2.15). What could this mean?
It seems obvious enough to me that "world" in this context doesn't refer to all human persons, or all human persons outside of the Church. That's not what he means, because the command not to love those persons doesn't make any sense. Christ offered himself on behalf the whole world in that sense (1 John 2.2), and Paul and John both make clear that God doesn't wait for a person to become a Christian in order to love them; rather he loves them even as they are still sinners (Rom 5.8; 1 John 4.10). Insofar as Christian ethics means assuming and developing a character in likeness to God's (Eph 4.22-4; Col 3.9-10), John cannot be suggesting that we ought not love people who are not Christians.
Likewise, John tells us that the commandment of God is this: that we should believe in his Son Jesus, and that we should love each other (1 John 3.23). Now it is obvious that the command to believe is given to a person who doesn't already believe. This means that the command to love one another is aimed at the whole world, at all persons, and not just as Christians. Christians have the obligation to love other Christians but also all people.
On the contrary, it seems to me that "world" in this sense refers to the culture and lifestyles and society of persons who live in sin. It doesn't refer to the persons themselves, but rather to the way that they live and the system in which they live, which is informed by sin and not by the truth of Christ. All of this is not to be loved, but the persons themselves are to be loved.
This introduces an interesting problem: how do you love the person, but not the lifestyle that they live, and the culture with which they identify? This is brought up in discussions regarding sexual ethics and gay marriage. How can you claim to love a gay person, and yet not to approve of their gay marriage, which is a fundamental component of the way they identify themselves?
Of course, we have the obvious counterexample: God loves us while we were still sinners, but he did not approve of or appreciate the things we did in our sinfulness, even if we identified with it. God loved Paul, but God could hardly have appreciated or approved of Paul's murderous zeal against Christians. Rather, it seems to me that the Bible teaches a different conception of identity than we are used to. Our identity is not something we create entirely ex nihilo, as if we start out blank slates. It must be that I am something beyond and prior to the life I decide for myself, the way I determine to live and the things I like to do and identify with.
What might this prior identity be? The obvious answer is drawn from the creation narrative in Genesis: the image and likeness of God (Gen 1.26-7). This is the true identity of the human person; this is what it truly means to be me, or you, or anyone else. Of course, in light of human sin, this identity is lost and we are convinced that actually we are something else. We construct various identities for ourselves, but inasmuch as these are not the image and likeness of God, they are a false identity. Rather, the true identity of the human person is demonstrated in Jesus Christ, who is the image of the invisible God (Col 1.15). Christ shows us what humanity truly is, and who we truly are.
Being a Christian means being like Christ (1 John 2.6). In light of the discussion above, we can say that being a Christian means discovering your true identity in Jesus Christ: he shows you who you truly are, beneath the cluttered surface that you might have constructed over time. What a human person truly is, Christ demonstrates: son of God, loving, benevolent, kind, good, righteous, fair, just, and so on. Everything is a false self that suppresses the image of God implicit in everyone.
Seiichi Yagi, in The Bible in a World Context, ed. Walter Dietrich & Ulrich Luz (Eerdmans, 2002), discusses the Christian philosophy of Katsumi Takizawa:
His basic thesis was: No matter what one is or is not, and unaffected by whether one has a religion, and which religion, one has an underlying foundation as the basis of being oneself. This is the primordial fact of "Immanuel," that is, "God with us." This primordial fact states that humanity is in God. Takizawa also called this primordial fact "unity of God and humanity" or "Christ." Not every human being is aware of this primordial fact. Only when one is awakened to it does a conscious religious life come about. Takizawa called this primordial fact "God's primary contact with humanity," and the religious life evolved by awakening to it he called "God's secondary contact with humanity" ... What is it in humanity that is awakened to it? Technically one could say: It is the human "I" that is awakened (p. 36).
We might assimilate this into the present discussion. Becoming a Christian means realizing the truth about one's status in the world; one's relation to God, who grounds our existence and keeps us in being from every passing moment; and although Takizawa would demur at this point, we can say that Jesus of Nazareth, as Christ, presents to us what it means to live in full awareness of this fact, in full awareness of the true identity of a human person.