The decision lies in our answer to the question – Do we, in the unknowable neighbour, apprehend and love the Unknown God? Do we, in the complete Otherness of the other – in whom the whole riddle of existence is summed up in such a manner as to require its solution in an action on our part – hear the voice of the One? We must, at this point, remember that our love of God takes place in an event when, in the ambiguity of our existence, a contrasted, unobservable 'Thou' presses upon us so insistently that we are confronted inevitably by the question, Who the am 'I'? Nor do we forget that the answer to the problem of the opposing 'Thou' lies in the question concerning this 'I', since we cannot but recognize that our most questionable 'I' is one with the 'Thou' by which we are confronted. And moreover, as we have already seen, it is in the ambiguity of the neighbour who fell among thieves that we encounter primarily and supremely the 'Thou' which is both question and answer. If I hear in the neighbour only the voice of the other and not also the voice of the One – that is to say, if I do not detect in him both question AND answer, – then, quite certainly, the voice of the One is nowhere to be heard.
Therefore – Thou shalt love thy neighbour AS THYSELF. Hidden and invisible is the neighbour, the other, whom, if I am really to love God, I must love as myself, to whom, that is to say, I am not and cannot remain another. In Christ – the turning point from question to answer, from death to life – I am not only one with God, but, because 'with God', one also with the neighbour. Love, then, is the 'spiritual relationship' (Kierkegaard) with the neighbour: that is to say, it is that unity – fellowship, communio – with him which is brought into being by means of the question and answer presented to me in the 'Thou' by which I am confronted, in so far as I am really one (fellowship, communio) with God. Who then, asks the scribe, is my neighbour? There is no escaping the answer – He that shewed mercy unto him that fell among thieves. Go and do thou likewise. Be thyself the neighbour; and there is no need for any further questions (Luke x. 29, 30-7). And so the neighbour is found to provide the answer to the question, Who, then, am I? and in this apprehension of him as the One who is 'Thou' and 'I' and 'He' lie the authorization and confirmation of our love towards God – whom we do not see.
Therefore – Thou shalt LOVE thy neighbour. Love is the relationship between men and their fellow men which is grounded – and therefore broken! – in the knowledge of God. In this relationship of love it is not men who confront men, but God who confronts God. Whether in this relationship we experience peace or conflict, whether, that is to say, love is what we are wont to call 'love', or whether it is not something more austere, far more bitter, is another question (xii. 9). However that may be, love is always the disclosing of the One in the other, in this and that and every other. Love, being altogether independent of its object – the concrete 'neighbour'! – is, for that very reason, altogether bound to it. Love beholds in every concrete neighbour only the parable of him who is to be loved; but nevertheless it does really see, it really does see in every temporal 'Thou' the eternal, contrasted 'Thou' apart from which there is no 'I' (xii. 3b-6a). Love, therefore, is love of men, of concrete, particular men; and it is this precisely because it has no pre-ference for any particular man. Love of the neighbour is love for him in his strange, irritating, distinct createdness and constitution, because and inasmuch as it undoes and loosens his constitution of createdness, as though it were a garment which must fall from his shoulders (Kierkegaard). Love is 'eternal, levelling righteousness' (Kierkegaard), because it justifies no man according to his desire. Love edifies the fellowship, because it seeks fellowship only. Love expects nothing, because it has reached the goal already. Love does not intend, because it has already done. Love asks no questions, because it already knows. Love does not fight, it is already the victor. Love is not the EROS that lusteth ever, it is the AGAPE that never faileth.
For this reason, then – Thou SHALT love thy neighbour. Understood strictly as the action of the NEW man, love is a duty; and as such, it is protected against caprice and disappointment and misuse. In this Thou shalt every divine Thou shalt not – not commit adultery; not kill; not steal; not covet (Exod. xx. 13-17, Deut. v. 17) – is summed up! In it the man who has been compelled to that 'not-doing' which is his turning back to God is once again impelled by God to action; cast down, he now again stands upright; a sinner, he now becomes righteous; dead, he is once more alive. In this Thou shalt! there is manifested the flaming sword of death and eternity. Therefore love is in itself perfect: it is the NEW doing, THE new doing, which is the meaning and fulfilment of all 'not-doing'. Love is the breath we breathe when, in the realm of evil, we have no breath left."
- Barth, Karl, 1968: The Epistle to the Romans. New York: Oxford University Press. S. 494-496