The first sentence of v. 4 can be taken in one of three ways; all of them make sense. (1) Conditional: ‘If you remain in me, I will remain in you’ (which is the assumption of the NIV rendering). Read in this way, the believer’s perseverance in remaining in Jesus is the occasional cause, not the ultimate cause, of Jesus’ remaining in the believer (cf. 8:31–32; 15:9–11). (2) Comparison: ‘Remain in me, as I remain in you’ (the Greek allows this: the second clause has no verb, but simply ‘and I in you’). The thought is coherent enough; the ‘and’ (as opposed to ‘as’) is mildly surprising. In the context of the threats on both sides of the verse, it is indefensible to take the ‘I in you’ as an absolute promise regardless of the perseverance or fickleness of the ostensible believer. (3) Mutual imperative: ‘Let us both remain in each other’, ‘Let there be mutual indwelling’. Again, however, the syntax is strange: the strong second person imperative in the first clause cannot easily be reduced to this mutual exhortation, and the normal Greek way of expressing this thought is by a hortatory subjunctive.
If the first reading, the conditional, has a slight edge, the general thought is in any case clear. No branch has life in itself; it is utterly dependent for life and fruitfulness on the vine to which it is attached. The living branch is thus truly ‘in’ the vine; the life of the vine is truly ‘in’ the branch. Lest the point be missed, Jesus steps away from the vine imagery a little and directly addresses his hearers (though he preserves the figure of ‘fruit’): Neither can you bear fruit unless you remain in me. This is not the inorganic growth of external accretion, like the growth of an alum crystal in an alum solution; it is organic growth, internal growth, driven by the pulsating life of the vine in the branch, and only this kind of growth produces fruit. The imagery of the vine is stretched a little when the ‘branches’ are given the responsibility to remain in the vine, but the point is clear: continuous dependence on the vine, constant reliance upon him, persistent spiritual imbibing of his life—this is the sine qua non of spiritual fruitfulness. The Christian or Christian organization that expands by external accretion, that merely apes Christian conduct and witness, but is not impelled by life within, brings forth dead crystals, not fruit.
Carson, D. A. (1991). The Gospel according to John (p. 516). Leicester, England; Grand Rapids, MI: Inter-Varsity Press; W.B. Eerdmans.