Summary: The sin of a nation reflects the spiritual temperature of the pulpit, inviting God’s judgement.
“Shall I not punish them for these things? declares the LORD,
and shall I not avenge myself
on a nation such as this?
“An appalling and horrible thing
has happened in the land:
the prophets prophesy falsely,
and the priests rule at their direction;
my people love to have it so,
but what will you do when the end comes?” 
February 21, 1781, was declared a day of fasting across England.  In response to the proclamation issued on that day, the Baptist divine, John Newton, preached a sermon entitled, “The Guilt and Danger of Such a Nation as This,”  from the text of JEREMIAH 5:29. Fearing the judgement of God, Newton warned his hearers to repent before it was too late.
The sermon Newton preached was pointed and pertinent, quite unlike many of the sermons that are delivered in this day. Here are some excerpts from that message. “The form of the question will not permit us to confine the application to Israel or Judah. The words are not, ‘On this nation particularly,’ but ‘On such a nation as this.’ The Lord, the Governor of the earth, has provided, in the history of one nation, a lesson of instruction and warning to every nation under the sun; and the nearer the state and spirit of any people resemble the state and character of Judah when Jeremiah prophesied among them, the more reason they have to tremble under the apprehension of the same or similar judgments. We likewise are a highly favoured people, and have long enjoyed privileges which excite the admiration and envy of surrounding nations: and we are a sinful, ungrateful people; so that when we compare the blessing and mercies we have received from the Lord, with our conduct towards him, it is to be feared we are no less concerned with the question in my text than Israel was of old.” 
“Though the occasion will require me to take some notice of our public affairs, I mean not to amuse you with what is usually called a political discourse. The Bible is my system of politics. There I read, that the Lord reigns; that he doth what he pleaseth in the armies of heaven, and among the inhabitants of the earth; that no wisdom, understanding, counsel, or power, can prevail without his blessing; that as righteousness exalteth a nation, so sin is the reproach, and will even totally be the ruin of any people… I hope we are now met, not to accuse others, but to confess our sins; not to justify ourselves, but to plead for mercy.” 
“The sin of a nation is properly the aggregate or sum-total of all the sins committed by every individual residing in that nation. But those may be emphatically called ‘national sins’ which, by their notoriety, frequency or circumstances, contribute to mark the character or spirit of one nation as distinct from another. It is to be hoped that some species of sins amongst us are not yet become national.” 
“Communities, as such, in their collective capacity, are visited and judged in the present life. And, in this respect, the Scripture considers nations as individuals; each having an infancy, growth, maturity and declension. Every succeeding generation accumulates the stock of national sin, and there is a measure of iniquity which determines the period of kingdoms. Till this measure is filled up, the patience of God waits for them, but then patience gives way to vengeance… When God is exceedingly displeased with a people, it is not necessary, in order to their punishment, that he should bury them alive by an earthquake, or destroy them by lightning. If he only leave them to themselves, withdraw his blessing from their counsels, and his restraint from their passions, their ruin follows of course, according to the necessary order and connection of causes and effects…”