Usually what theist argue as proof for existence of god is how intricately and precisely designed, everything in the universe is.
For example - The availability of a complicated pumping system, the human heart, A multitasking super computer, the brain, and various other factors, like being endowed with all the essential number of muscles and nerves. It all seems like intellegent design, like how a mechanic designes a car, and even the accuracy of the laws of nature (a small change in gravitational force would cause the collapse of the universe), have also been used to support for their cause, how did something as complicated as the universe, that has millions of probabilities of going wrong, from its creation till now working without an all powerful entity?, is their question.
People like Richard Dawkins have argued that, only because these seemingly "perfect" situations exist, we do, it makes perfect sense.
For example.
An algae cannot grow if there is no moisture.
We would not have existed if there was no oxygen or water or carbon were already here.
They were not put here for us, we exist because they are here. If there weren't here, some other species might have evolved or earth would have remained barren like mars or Venus.
If it is all intelligent design why are there defects in babies, if it is intelligent design then all the babies would be checked and tested before they are sent to this world, like how we check our cars.
We exist, because we have all necessary conditions for us to exist, not the other way around. There were millions of species which have failed to survive, because they weren't good enough to survive here. In the same way there might have been millions of failed planets, failed galaxies, failed universes and failed big bang, but we don't know because we weren't there. We are just a probability of some mysterious event, which also has multiple other probabilities, which caused many random probable sequences, not because of an all powerful organism living in the clouds. .
