To understand Niebuhr (the brother of Richard) requires a note that he
reached adulthood soon after the First World War ended in 1918, at a time during
which the theological liberalism of Schleiermacher and his disciples was under
severe attack. Disillusion in America with the (mainly European) liberal
tradition arose in the post-war generation. As in Europe many began asking how
such terrible behaviour was possible if man was supposed to be inherently good
and progressing steadily towards perfection.
Niebuhr was born in Wright City, Missouri, in the United States. At
theological college he was introduced to the work of Adolf von Harnack. He
was ordained Pastor of the Evangelical Reformed Church, serving in the
Bethel congregation Detroit, until 1928. He was then appointed Professor
of Ethics at the Union Theological Seminary, New York.
His time in Detroit coincided with the expansion of the motor industry.
Niebuhr was troubled by what he judged to be the demoralizing effects of
industrialism on the workers. He criticised the Ford motor company for what he
saw as inhumane conditions created by the assembly lines combined with lack of
security. He allowed union organizers into his pulpit to promote workers'
Niebuhr was at first an ardent pacifist. A visit to Germany in 1923
reinforced these views. In 1931 he and others founded the Fellowship of
Socialist Christians. But, like many others of the time who learned of
Stalin's prison camps and mass killings, Niebuhr became disenchanted with
socialism. He is reputed to have voted for the Liberal Democrats at the
outbreak of World War II. About then he also abandoned pacifism, believing
that the war was justified to preserve Western democracy. By 1953 (in
Christianity and Power Politics) he wrote that
... pacifism either tempts us to make no judgements at all, or to give an
undue preference to tyranny.
H S Wieman suggests that the central theme of Niebuhr's teaching can be
summarised as follows:
... a divine, forgiving, and timeless love "beyond history" gives meaning
to human life. 
His thinking can be seen as a reaction against the non-involved Protestant
theology of the time. In practice, if not in theory, it held that the
Christian ministry was more or less entirely carried out through the act of
preaching. Non-involvement in social issues was a norm. Niebuhr was critical
of those who constantly emphasised Christianity's transcendent elements while
remaining indifferent to world affairs. He thought that Karl Barth was
particularly at fault in this respect.
The theological movement initiated by Karl Barth has affected the thought
of the Church profoundly, but only negatively; and it has not challenged the
thought outside of the Church at all. 
In line with his social concerns, Niebuhr thought a good deal about ethics.
In 1932 he wrote Moral Man and Immoral Society. He suggested that "...
reason is always the servant of interest in a social situation ..." A
biologist might put the same thing in terms of irresistible survival
instincts. His position in effect means that group "morals" - what are better
called customs or norms, I think - always prevail over individual choices
between right and wrong. (In later years he thought he might have been wrong
about this.) It means also that reason gives way to selfishness as
expressed by society in a collective egoism.
In every human group there is less reason to guide and to check impulse,
less capacity for self-transcendence, less ability to comprehend the needs
of others [and] therefore more unrestrained egoism than the individuals, who
compose the group, reveal in their personal relationships ... All social
co-operation on a larger scale than the most intimate social group requires
a measure of coercion ... Every group, as every individual, has expansive
desires which are rooted in the instinct of survival and soon extend beyond
it. The will-to-live becomes the will-to-power ... Thus society is in a
perpetual state of war.
In place of the so-called "social gospel" of Schleiermacher (not quite the
same thing as the social gospel of the mid-60s and 70s of the 20th century)
Niebuhr later evolved a type of theology he called "Christian Realism" (in
Nature and Destiny of Man, 1941). Christians should be politically realist
by clearly recognising the powers at work in society - typified by the masses
and by privilege and wealth. One way of engaging these powers is by skillfully
transmuting Christian values into an independent ethic and applying that ethic
to the dynamics at work in society. But this can be done effectively only if
one is a realist about the limitations of traditional theology. For Christian
faith to be truly energising it must be in constant interaction with the
culture of its day, borrowing and rejecting according to its best judgment.
This emphasis sprang from a concern with social analysis - which some
called "theological anthropology". Through this it was hoped to penetrate
below and behind the easy ideals of "love" and "justice" which had been the
wellsprings of what some thought of as the wishy-washy, over-idealistic
liberal theology of pre-war socialism. Thus religion rather than reason
provides a way ahead.
The religious sense of the absolute qualifies the will-to-live and the
will-to-power by bringing them under subjection to an absolute will.
We are all, said Niebuhr, biased by our positions in society and by the
historical processes from which we spring. These distortions can be reduced by
a personal faith in a God of love and mercy, whose great plan for creation
makes human plans look puny. With a faith-mediated perspective, we are enabled
to see life more clearly and realistically. Our social judgements are rendered
less subject both to unrealistic assessments of our significance, and to undue
pessimism that things can't get better. Niebuhr appears to me never to have
entirely reconciled reason and religion (as he defined them). At the same time
he did give reason a large role in the practicalities of justice.
Harmonious social relations depend upon the sense of justice
as much as, or even more than, upon the sentiment of benevolence. This sense
of justice is a product of the mind and not of the heart. It is the result
of reason�s insistence upon consistency
In rejecting liberal political idealism, which Niebuhr perceived as
political innocence in the light of the Great War and the rise of German
totalitarianism, he proposed that all institutions are shaped by power. This
power injects self-interest into political processes. As a result we tend to
wrongly confuse the needs of others with our own. He produced a well known
saying along these lines:
Man's capacity for justice makes democracy possible, but man's inclination
to injustice makes democracy necessary.
One implication of Niebuhr's position was to put on the spot those who
could not acknowledge that idealism is no substitute for successful action.
Empty talk is fine - but it has to issue in a practical program of change. And
any such program which does not take full account of the human capacity for
ill will fail - in essence a reflection of the American pragmatism which was
so powerful a philosophy between the two world wars.
One might ask where in all this is the theology. One answer is that Niebuhr
(without perhaps quite enough care for detail) did refer his thinking back to
biblical narratives and especially to the gospels. He thought that the latter
present the same duality of human nature he saw in the 20th century. Man is
without doubt finite and limited in many respects. And yet, as the Bible makes
plain, man is also created in God's image. He is bound by the limits of his
historical situation and perspective, and yet capable of transcending them.
He compared theology to a painter. The painter's skill lies in depicting
three dimensions using only two. In a similar process the theologian tries to
speak of God. The thought forms of creation have to be used to describe God.
But God is transcendent. By definition, the transcendent can't be described.
So all theology is also by definition a gross approximation. It is, I suppose,
rather like painting with black paint in a darkened room. Or, as Richard
Holloway puts it, like reading a map of a territory which doesn't exist. Those
who think in terms of drama and history, said Niebuhr, are likely to
understand more than philosophers and theologians who try to straightjacket
everything with reason.
What this amounts to is a quite sharp distinction between the kinds of
things which can be rationally thought through and religious beliefs which
can't be rationally defended. The first struggles with practical social
problems. The second is more mystical (Niebuhr talked a lot about "myth") -
what might best be described as a theoretical superstructure which lies beyond
the tests of reason.
Thus the Christian faith has the power to motivate action:
By its confidence in an eternal ground of existence which is,
nevertheless, involved in man's historical striving to the very point of
suffering with and for him, this faith can prompt men to accept their
historical responsibilities gladly.
This historical responsibility is intensely situational in the sense that
no measure of social justice can escape the restraints of history. Social
action varies depending upon what history lies behind it. Wide social freedoms
may be required in one situation, and narrower freedoms in another.
The power processes of the world are obviously stronger than Christian love
(agape). But what God has done is to reveal this love to us through
Jesus of Nazareth. Niebuhr proposes that a "hidden Christ" operates in
history. Through him we know that God's ultimate purposes for the world
transcend history and therefore worldly power. But we mustn't think that only
Christians have access to this source of faith. It might be that
non-Christians reach a more genuine repentance and humility.
Niebuhr thought that the fate of Jesus and the facts of society as he knew
them meant that Christian love leads inevitably to the equivalent of death on
a cross. The consistent selflessness required by such love is beyond most of
us - though it does appear and succeed occasionally in the broad sweep of
Justice, not Christian love, is the principle which regulates human society
for good. Conflict springs up as groups compete to satisfy their
self-interest. This means, he thought, that social processes necessarily
involve a degree of coercion. The ideal of total mutual co-operation may lie
somewhere in the future. But disaster will follow if it is attempted now. In
1932 he wrote:
It is because men are sinners that justice can be achieved only by a
certain degree of coercion on the one hand, and by resistance to coercion
and tyranny on the other hand. 
By 1968 he was able to write that
Man's concern for some centuries to come is not the creation of an ideal
society in which there will be un-coerced and perfect peace and justice, but
a society in which there will be enough justice, and in which coercion will
be sufficiently non-violent to prevent this common enterprise from issuing
in complete disaster. 
Because we are somewhat short-sighted about God's greater plan for us, we
tend to get anxious. From that anxiety arises sin. The latter takes various
forms. Some struggle in vain to control and direct the surging tides of
history. Some curry favour by bribing or petitioning God. Others try to dull
the pain of anxiety by the false balm of sensual enjoyment. Sin lies in
turning away from God into self-centredness. In effect, we tend to set
ourselves up as absolutes in place of God.
There is a pride of power in which the human ego assumes its
self-sufficiency and self-mastery and imagines itself secure against all
vicissitudes. It does not realise the contingent character of its life
and believes itself to be the author of its own existence, the judge of its
own values, and the master of its own destiny. 
Though humanity is inherently sinful, people are also intensely free. We
are fully able to transcend our nature and to influence history (though not to
control it). The fact that we know that we are not always totally free is sure
evidence of our freedom. We are not, insists Niebuhr, totally corrupt as
suggested by Karl Barth and others.
The emphasis [by Barth] upon the difference between the holiness of God
and the sinfulness of man is so absolute that man is convicted not of any
particular beaches against the life of the human community but of being
human and not divine. 
Reason may be, according to Niebuhr, a somewhat blunt instrument. It can be
used for good or evil. It serves us poorly when we attempt to impose coherence
on the world and then reject what doesn't fit that imposed pattern. If we
trust in reason rather than in God's forgiving love, we sin.
Even though Niebuhr didn't go all the way with Barth, his approach turns
out to be remarkably similar. My guess is that Niebuhr failed to harmonise
traditional doctrines with down-to-earth socialism. The former inevitably
rests upon a dualistic world, the latter upon a material world. Barth dealt
with this by proposing that reason can take us only so far. Then "faith" has
to take over - and that's what Christianity is all about. Niebuhr's failure is
not surprising. Perhaps it is impossible to achieve a true synthesis of
traditional Christianity with social justice.
Niebuhr has nevertheless proved influential in relation to the growth of a
social awareness amongst Christians since World War II. He appears to have
provided many with a framework through which to come to terms with the obvious
shortcomings of traditional Christianity with regard to social action. But it
should noted that his approach is dualistic. God's love and providence operate
on this world from a perfect or at least better "outside" reality.
 The Encyclopedia of Philosophy, Macmillan, 1967
 The Nature and Destiny of Man, Vol 2 quoted by J Macquarrie in
Twentieth-Century Religious Thought, SCM Press Ltd, 1963
 Moral Man and Immoral Society, 1932
 The Relevance of an Impossible Ethical Ideal in From Christ To
The World, ed. Wayne G. Boulton, Erdmans, quoted by I I Imsong in
Reinhold Niebuhr and Christian Realism, 1999