2:15 Do not love the world or the things in the world. If anyone loves the world, love for the Father is not in them (Μὴ ἀγαπᾶτε τὸν κόσμον μηδὲ τὰ ἐν τῷ κόσμῳ. ἐάν τις ἀγαπᾷ τὸν κόσμον, οὐκ ἔστιν ἡ ἀγάπη τοῦ πατρὸς ἐν αὐτῷ). After affirming his confidence in the genuine Christian faith of his readers, John here issues the first of ten imperatives in the letter (also in 2:24, 27, 28; 3:1, 7, 13; 4:1 [2x]; 5:21). He has previously said that the one who loves a brother or sister abides in the light (2:10), so this command stands in sharp contrast, creating a new category in the Johannine duality, the "world" (κόσμος). Living in the light means a life of love for God and for fellow believers; love for the world is excluded from living in the light. In fact, love for the world as John defines it is mutually exclusive with love for the Father (see "In Depth: The ‘World’ in John’s Letters" at 2:16).
Because love for the world is syntactically parallel with "love for the Father" (ἡ ἀγάπη τοῦ πατρός), the genitive is most likely objective, referring primarily to the believer’s love for the Father, which is expressed by loving others according to the will of God. This expression of love for God achieves the goal of God’s redemptive love for his people (see comments on 2:5a-b). Therefore, those who love God must not love "the world," for by John’s definition the world is all that is in rebellion against God. Clearly "love" for the world is of a qualitatively different kind than the love one is to have for a brother or sister, which is an expression of care and concern. Here, "love" refers to an attraction to something that one wishes to enjoy, an indulgence in things that are not in the light. It is to want to participate in what is set in rebellion against God.
