tl;dr: the term "act of God" has been around for thousands of years. Translated several times. It wasn't until only a couple hundred years ago that the term "God" was ever introduced into the phrase. And even then, it was decreed by Judges of the time that there is no religious backing to the phrase.
Long detailed version for anyone who is interested in the etymology of the phrase.
"Act of God" is a traditional term used in contract law that has existed for millennia, dating back to Roman law. Romans originally used the term 'vim maiorem' in Latin which meant "Superior Force". Contracts using this term exist as far back as 509BC. 'Vim maiorem' became standard contract terms in most Roman contracts found from this period. It released a person from liability based on actions that were unpreventable or unforseeable.
As ancient Rome fell, Italy began to disperse and adopted the same term 'vim maiorem' into the Italian language. This well-understood concept and term enabled contracts to be made among dispersed Italian villages. The tradition of this latin phrase stayed alive in italy for another thousand years until the renaissance.
During the renaissance, the French were looking to rebuild a great society like the Romans and determined that law was a critical part of the culture. Contracts became commonplace again, and the French were inspired by the contracts written between these Italian cities. They found the 'vim maiorem' term helpful is absolving liability for unforeseeable circumstances. They translated the term to "force majeure". This literally translates into 'the greatest force'.
Again... no spiritual component.
The tradition later expanded into England in the 16th century. While the English didn't like the French, they did like the concept of "Force Majeure". But they didn't want to use a French term. So they originally adopted the term 'vis maior' in contractual law, which was an alternate latin term used along with 'vim maiorem' which translated closer to "major acts'.
Thomas Wilson is the English man who translated "vis maior" into the English term "Act of God". He felt that "the Italianated [words] counterfeited the great Kinges Englishe" [sic]. As a respected judge, he sought to find an appropriate English translation that gave respect to the King instead of giving credit to the French or Italians. As Thomas Wilson was a Christian who was heavily engaged in the Christianity revolution in medieval England at the time, he coined the term "Act of God" to replace "vis maior".
This is the first time that this traditional phrase carried any religious weight. But that relgious weight was quickly dispelled by future court rulings.
> "Judges continued [throughout the 17th century] to rule that in law, an act of God did not depend on divine influence, including violent storms at sea, unprecedented rainfall, extraordinary floods, earthquakes, and death. In 1609, a British court ruled that a fire caused by lightning was an act of God. In 1785 a court ruled that a fire NOT caused by lightning was NOT an act of God. In 1886, England's Lord Esher ruled, 'In the older, simpler days I have myself never had any doubt but that the phrase does not mean act of God in the ecclesiastical and biblical sense... but that in a mercantile sense'. By the 1800s, the courts routinely rejected claims that God was responsible for human negligence.[1]"
At this same time, contracts were being passed from England to the newly formed USA. Most of the founding fathers were lawyers by trade and "Act of God" quickly spread as a legal term in the US.
"Act of God" is defined as "any accident, due directly and exclusively to natural causes without human intervention, which could not be prevented."
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1380373/?page=4