Augustus
…
If you read the wiki article you know it's a controversial question. Some take my side, others take yours.
If it is a controversial question why are you so confident he was 'definitely not a believer' though? What is the evidence or scholarship you are basing this on?
On basically every historical issue you can say "scholars disagree", but that doesn't make all of their arguments equally common or equally plausible based on the evidence. Those arguments go back to the fiercely anti-Christian Edward Gibbon in the 18th C who is the source of nonsense like the Christians destroying the Great Library of Alexandria and causing the downfall of the Roman Empire. This school of anti-religious Enlightenment era history has generally been shown to be wrong on all kinds of things by modern historians.
Given your concern for evidence, surely you must have a rational argument as to why he carried out a very elaborate conspiracy to 'adopt' a religion with so many disadvantages and risks and so few apparent benefits, spent so much financial and political capital on it, as to why his children turned out to be Christians, why his nephew, the last Pagan Emperor Julian 'the Apostate' was raised Christian believed Connie was Christian, etc.? In your opinion, what makes it even probable he wasn't Christian, let alone definite?