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Read and understand the five proofs of St. Thomas Aquinas. Here is an extremely TLDR version in pic related.
Let's not focus on Christianity or a personal God for the moment.
Do any serious people here actually disagree with the philosophical necessity of a prime mover?
Here is a very simple and all encompassing thought: the universe exists. The only possible way for the universe to exist is for something with the properties of a prime mover to bring it into existence. Either that or the universe itself has those properties, but I find that not to be a very compelling argument.