Atheist: "The beginning of the universe by definition was a natural event."
Answer: To claim the beginning of the universe is by definition a natural event is what is known as “begging the question”. Begging the question is a type of informal fallacy in which an implicit premise (NATURALISM IS AN ABSOLUTE TRUTH) would directly entail the conclusion (THE CAUSE OF THE UNIVERSE) ; in other words, basing a conclusion (THE CAUSE OF THE UNIVERSE WAS A NATURAL EVENT) on an assumption (NATURALISM IS TRUE) that is as much in need of proof or demonstration as the conclusion itself.
The beginning of the universe was the beginning of space, time, and matter (natural events).
A “natural” event implies something that is caused within the boundaries of space time. But if the beginning of the universe was the beginning of space, matter, and time itself then the cause couldn’t have been “natural”. In other words, your assertion is stating what caused the natural world was the natural world. Any assertion that a thing can make itself is self-contradictory. This is because in every case where something has actually been made, that which caused it existed prior to it. For example, an oak tree may have found its immediate cause in the planting action of a pre-existing squirrel and by the acorn production of a pre-existing oak tree. So, for the universe to have made itself, it would have had to exist prior to its existence--a contradiction of the undeniable first principle of causality.
The beginning of the universe was the beginning of space, time, and matter (natural events).
A “natural” event implies something that is caused within the boundaries of space time. But if the beginning of the universe was the beginning of space, matter, and time itself then the cause couldn’t have been “natural”. In other words, your assertion is stating what caused the natural world was the natural world. Any assertion that a thing can make itself is self-contradictory. This is because in every case where something has actually been made, that which caused it existed prior to it. For example, an oak tree may have found its immediate cause in the planting action of a pre-existing squirrel and by the acorn production of a pre-existing oak tree. So, for the universe to have made itself, it would have had to exist prior to its existence--a contradiction of the undeniable first principle of causality.