In this paper is analyzed the fragment DK 88, B25 as a source of atheism and unbelief in Ancient Greece. After a description of the different studies that have been done of the fragment, we should focus the analysis in the different ideas and philosophical positions of the fragment. A detailed study of the text shows us many of the philosophical theories that compose the thought of the midfifth century BC. Those ideas are developed from the natural theology that is (...) constructed in theistic opposition to the theology of the poets. In the mid-fifth century BC, specifically in Athens, this natural theology is expanded towards the Athenian intellectual circles that relativize the ritual practice and the existence of the gods. This is the cause of the irreligious and atheism positions that we can see in the fragment DK 88, B25. (shrink)