Many years ago I was commissioned to write a short book on Atheism. I had already written a short What is Philosophy? [ republished at my www.selectedworks.co.uk site]and thought I could deliver on Atheism.
But I couldn't. I found it emotionally difficult and I had other (emotional) worries, notably a divorce.
You could, for starters, distinguish three kinds of atheism : ontological, epistemological and moral.
Thinking about what there is (ontology), you can argue that there is no need to postulate a god to explain the universe and so, using Occam's Razor [ Don't imagine more things than you have to ], don't postulate one. You can also argue that any well-founded ontology looses its robustness or coherence once you start adding a god into it. And so on. None of this interests me very much, though I read some of the books.
Thinking about how we know things (epistemology) you can argue that it's unknowable whether there is or isn't a god - which leads to agnosticism rather than atheism - or that there is no evidence pointing towards the existence of a god or that the evidence points the other way. For example, the Problem of Evil (the theodicy problem) suggests that even if there is a god, he, she or it isn't a good one or isn't a powerful one. There's too much evil in the world. This is a bit more interesting and I read quite a few books.
But my own brand of atheism, such as it is, says that it is wrong to believe in god. It's a moral question. I ended up with the following formulation: If a good god did exist, he would not wish us to believe in him any more. Too many crimes have now been committed in his name. To expand on that, there is now something indecent about believing in god. From a slightly different angle, there is something weak-willed about believing in god. It is too closely connected to hedging one's bets, since in general (though for no compelling reason) belief in god and belief in personal immortality are interlinked. Take away the promise or threat of immortality and there is not much left of Christianity. The Vatican would not be able to frighten anyone without immortality.
But whether or not someone is a theist does not trouble me very much. In fact, from the religious and theological literature I read from my teenage years on, I always came away with respect for those who live quietly pious lives - I say "quietly" because the lack of demonstrativeness is the core of the piety. And I think there can be non-theist versions of that piety: paying attention [ to someone, to something ] is the natural piety of the soul.
What does trouble me is organised religions. With very few exceptions, they are dreadful outfits - mean spirited, cruel, corrupt, self-indulgent, full of hate towards women and children. The Vatican - a totalitarian bureaucracy - has demonstrated all that, continuously, for centuries.
So I want to clip the wings of organised religions and keep them out of public life. No state religions, no faith schools, no NHS hospital chaplains, no red carpets for the big wigs, no tax breaks, no immunity from civil and criminal law.
That makes me a secularist.
I am surprised how weak we are in our dealings with organised religion. Mussolini granted the Vatican recognition as a "state" because its bureaucrats wanted to put themselves beyond the reach of ordinary civil and criminal law. That is what the 1929 Lateran Pact is all about. Time to repudiate it. Send in the tanks. No Vatican State, just a church whose bureaucrats, like all other citizens, are subject to the laws of the country in which they live.