Diagram of the simplest known self-reproducing life form on Earth.
Naturalism is the idea that life on earth arose spontaneously, without outside intelligent agency, from simple proteins combining to form a living cell, able to reproduce itself, and able to evolve over time into the myriad of life forms we see today.
This is clearly a big ask for simple proteins and a big ask for any level of intelligence to arrange.
For example, humans are unable to do this process themselves today, given our vast knowledge and scientific acumen. We can only manipulate existing living forms, not create them from scratch from proteins.
Therefore, to leave such an operation to blind chance, given the revealed complexity of living cells, the simplest of which is said to be more complicated than our most sophisticated space craft, seems to be way beyond belief.