One of the most profound writings about religion was done by Tom Paine in his "Age of Reason, Part I and II".
Teddy Roosevelt called him "that dirty little atheist". An excerpt:
I believe in one God, and no more; and I hope for happiness beyond this life.
I believe the equality of man, and I believe that religious duties consist in doing justice, loving mercy, and endeavoring to make our fellow-creatures happy.
But, lest it should be supposed that I believe many other things in addition to these, I shall, in the progress of this work, declare the things I do not believe, and my reasons for not believing them.
I do not believe in the creed professed by the Jewish church, by the Roman church, by the Greek church, by the Turkish church, by the Protestant church, nor by any church that I know of. My own mind is my own church.
All national institutions of churches, whether Jewish, Christian, or Turkish, appear to me no other than human inventions set up to terrify and enslave mankind, and monopolize power and profit.
I do not mean by this declaration to condemn those who believe otherwise; they have the same right to their belief as I have to mine. But it is necessary to the happiness of man, that he be mentally faithful to himself. Infidelity does not consist in believing, or in disbelieving; it consists in professing to believe what he does not believe.
It is impossible to calculate the moral mischief, if I may so express it, that mental lying has produced in society. When a man has so far corrupted and prostituted the chastity of his mind, as to subscribe his professional belief to things he does not believe, he has prepared himself for the commission of every other crime. He takes up the trade of a priest for the sake of gain, and, in order to qualify himself for that trade, he begins with a perjury. Can we conceive anything more destructive to morality than this?
Soon after I had published the pamphlet COMMON SENSE, in America, I saw the exceeding probability that a revolution in the system of government would be followed by a revolution in the system of religion. The adulterous connection of church and state, wherever it had taken place, whether Jewish, Christian, or Turkish, had so effectually prohibited, by pains and penalties, every discussion upon established creeds, and upon first principles of religion, that until the system of government should be changed, those subjects could not be brought fairly and openly before the world; but that whenever this should be done, a revolution in the system of religion would follow. Human inventions and priest-craft would be detected; and man would return to the pure, unmixed, and unadulterated belief of one God, and no more.
CHAPTER II - OF MISSIONS AND REVELATIONS.
EVERY national church or religion has established itself by pretending some special mission from God, communicated to certain individuals. The Jews have their Moses; the Christians their Jesus Christ, their apostles and saints; and the Turks their Mahomet; as if the way to God was not open to every man alike.
Each of those churches shows certain books, which they call revelation, or the Word of God. The Jews say that their Word of God was given by God to Moses face to face; the Christians say, that their Word of God came by divine inspiration; and the Turks say, that their Word of God (the Koran) was brought by an angel from heaven. Each of those churches accuses the other of unbelief; and, for my own part, I disbelieve them all.
As it is necessary to affix right ideas to words, I will, before I proceed further into the subject, offer some observations on the word 'revelation.' Revelation when applied to religion, means something communicated immediately from God to man.
No one will deny or dispute the power of the Almighty to make such a communication if he pleases. But admitting, for the sake of a case, that something has been revealed to a certain person, and not revealed to any other person, it is revelation to that person only. When he tells it to a second person, a second to a third, a third to a fourth, and so on, it ceases to be a revelation to all those persons. It is revelation to the first person only, and hearsay to every other, and, consequently, they are not obliged to believe it.