This is primarily for
@cjab, though of course anyone can participate. I will start by summarizing the main points Russell raises in his well-known short essay
Why I am not a Christian. The full text can be read
here or
here. It should be noted that this was originally delivered as a speech to a general audience, and is accordingly often humorous in tone and not as philosophically rigorous or technical as a written article might be.
What is a Christian?
Russell begins by defining terms, and concludes that three things are minimally necessary for qualifying as a Christian: Belief in God and immortality, and that Jesus was at least the best and wisest of all men. These therefore will be the targets of his criticism.
The Existence of God
While acknowledging that the list is not complete, Russell considers 5 classical arguments for God. The first is
The First Cause Argument which he argues cannot have any validity on account of begging the question of what caused God. He says "There is no reason why the world could not have come into being without a cause; nor, on the other hand, is there any reason why it should not have always existed. There is no reason to suppose that the world had a beginning at all."
He next considers
The Natural Law Argument, which is the idea that God is revealed by the regularity of nature. Russell notes that the simple laws of Newton have been replaced by the less intuitive theories of Einstein and the statistical averages of QM, less suggestive of design vs chance, and then explains the difference between prescriptive human laws and descriptive natural laws, where only the former imply a law-giver. He also points out that God's choice of laws would be either arbitrary or subject to laws independent of God.
The third argument considered is
The Argument from Design, and Russell argues that evolution has largely undercut this by showing how organisms have adapted to fit their environment rather than having the environment tailored to fit them. He also observes that this world is far from the perfection unlimited omniscient design could be expected to produce, especially given that the solar system and the universe itself will eventually tend towards conditions making life impossible.
Fourth is the category of
Moral Arguments for Deity, which Russell attributes primarily to Kant and rebuts with Euthyphro's Dilemma, arguing that either God's moral dictates are arbitrary meaning God cannot be non-trivially 'good', or God is himself subject to morality and therefore not the source of it.
The final considered argument is what he calls
The Argument for the Remedying of Injustice, which is the idea that justice requires an afterlife where the injustices of our known world can be redressed. Russell rebuts this by saying it is as illogical as seeing rotten apples at the top of a crate and assuming there must be lots of good ones underneath to redress the balance.
He also observes that these arguments are rarely what actually motivates belief in God, which is more often due to childhood indoctrination and the desire for there to be someone powerful looking out for us.
The Character of Christ
Russell points out that few Christians take Christ's maxims seriously, such as turning the other cheek, which predates Christ anyway; his injunction against judgement, which hardly any Christian follows; and his command to give away one's belongings to the poor. These points Russell commends as good, if hard to live up to, before moving on to those teachings from Christ which he cannot agree with.
Defects in Christ's Teaching
Russell observes that we cannot know that Christ as depicted in the Gospels ever really existed, but argues that if he did then he cannot be considered the best and wisest of all men. The first reason given is that Christ appeared to believe, quite wrongly, that his second coming was imminent and would occur within the lifetimes of those he addressed.
The Moral Problem
A more significant failing in Christ's teachings is his belief in hell (Matt 23:33, Matt 12:32, Matt 13:41-42), and Russell compares his indignation towards doubters unfavorably with the calmer attitude of Socrates. He says "I think all this doctrine, that hell fire is a punishment for sin, is a doctrine of cruelty" and one which has caused a lot of unnecessary suffering.
The Emotional Factor
Russell then considers the claim that we must refrain from criticizing religion because people would become evil and immoral without it, against which he argues that the religious have been equally cruel, that the cruelty of a society has tended to correlate with its religiosity, and that almost all moral progress has been made against the opposition of organized religion.
How the Churches have retarded Progress
Russell further argues that this is still the case today, as religion continues to cause suffering and impede progress by choosing "to label as morality a certain narrow set of rules of conduct which have nothing to do with human happiness", focusing instead on making people fit for heaven - and thereby quite unfit for the real world.
Fear for the Foundation of Religion
Russell diagnoses religion as founded upon fear - of death and the unknown - which explains why it so often leads to cruelty. He instead advocates science as a foundation for overcoming fear and making the world a better place.
What We Must Do
Russell concludes on a positive note: "We want to stand upon our own feet and look fair and square at the world - its good facts, its bad facts, its beauties, and its ugliness: see the world as it is, and be not afraid of it. Conquer the world by intelligence... A good world needs knowledge, kindliness, and courage; it does not need a regretful hankering after the past, or a fettering of the free intelligence by the words uttered long ago by ignorant men."