As an example, here is what I believe:
Origin of the universe -
I believe that mass/energy is eternal, and that the space-time it occupies has passed through at least one singularity, the most recent of which we call the Big Bang.
This is a 'soft' belief - I think it is true only as a placeholder while I await actual evidence, and I expect to revise it if any such evidence is found. It is, I believe, the simplest hypothesis that fits with the evidence we currently have; But the nature of a singularity is that we really can't know for sure what happened at or before that point - or even whether 'before' is a valid concept in this context.
Origin of the Earth -
The Earth is a bit of heavy element 'slag' left over after the gravitational collapse of the gas cloud that formed the Sun. That gas cloud was the partial remains of several Generation I stars, which generated heavy elements via end of life processes, probably including supernovae and neutron star collision. Conservation of Angular Momentum in the collapsing cloud of gas led to rapid rotation of the proto-sun, and a number of small planets and some other debris span off and became planets, asteroids and comets. The inner planets lost most of their light elements due to solar wind, leaving the Earth as a ball of iron, surrounded by a mantle of mid-weight elements, mostly oxygen and silicon. Cometary bombardment returned some of the lighter elements to the surface of the Earth in the form of water.
I am pretty confident in this belief, as it is backed by a lot of evidence; But I am not an expert in the field, and may well have some mistaken beliefs here, so I am happy to revise this understanding if presented with better evidence. The details are likely out of date, but the basics are close enough to reality that I consider it highly unlikely that massive revisions of this belief will be needed.
Origin of life -
Aqueous solutions containing the elements carbon, nitrogen, oxygen, sulphur and hydrogen, plus traces of other mid-weight elements, energized by solar radiation for several hundred million years, naturally give rise to trillions of different structures and chemical combinations. At some stage, some stable configurations are inevitable; And equally inevitable in a high energy flux environment due to solar radiation is that these stable structures will be able to transfer and transform energy. The development of simple energy cycles is inevitable, because those structures that fail to do so will break up and be reformed into new structures. Once a set of cyclic reactions exists that dissipates energy without falling apart, and which generates copies of itself as part of the energy dissipation process, evolution by natural selection is unavoidable; And we call cyclic reactions that dissipate energy without falling apart, and which generate copies of themselves as part of the energy dissipation process, 'life'.
I am very confident in this belief, as I have studied much of the detail (and we assuredly do NOT have room for even a small fraction of it here) in my days as a student of Molecular Biology. I am certain that the full and precise details are not yet known, but I am equally certain that this basic sketch is not likely to be wrong in any significant way. I would change my mind if presented with really solid evidence that this was wrong, but it seems very unlikely indeed that any such evidence could exist, but not yet have been found.
If you do want to look into the details of any of these things, then the place to look is at:
https://scholar.google.com.au/; However I would recommend that you first study Physics, Chemistry and Biology to at least undergraduate level first, or the details will likely be beyond your grasp.
All of the above beliefs are independent of my atheism, and all of the above are believed by some (but not all) atheists, and by some (but not all) theists. There are non-trivial numbers of theists who believe that the above describes the mechanism by which a god or gods created these things. Personally, I don't see the need for any gods to explain any of this, nor any evidence that they are anything other than fiction written by people who were ignorant of both the scale and origins of all of these three phenomena.
Al-‘Uzzá, Arabian goddess of fertility.
