The civil war between the Imperials and Stormcloaks is one of the major story beats in The Elder Scrolls V: Skyrim, and the conflict was in part brought about by the Thalmor's outlawing of Talos worship. Although the reasons behind this law aren't explicitly spelled out in Skyrim's main questline, there's quite a bit of lore and history that explains why the Thalmor would ban worship of the human god, as well as how it aligns with their goals for the future of The Elder Scrolls' world.
In-universe, Talos is one of the more controversial figures among The Elder Scrolls' Divines. Originally, Talos was a mortal man named Tiber Septim, largely viewed as a war hero among humans. He was responsible for uniting all of Tamriel under a single Empire and, according to what's currently known about The Elder Scrolls' timeline, was the first person to successfully do so. It's generally believed Tiber Septim then went on to achieve CHIM, one of many methods of achieving divinity in The Elder Scrolls, elevating himself to godhood and taking his place with among the Nine Divines.
The legends surrounding Talos are disputed, however. Some of Tamriel's people believe Talos' godhood was a reward for his actions as a mortal, while others - particularly of the High Elven Aldmeri Dominion - reject Talos' divinity entirely. Although some fans theorize this is solely due to the elven races resenting the possibility of a human achieving divinity, Skyrim offers a few other explanations that help flesh out the High Elves' banning of Talos worship.
How The Thalmor Use The Talos Ban To Divide Skyrim
Firstly, it's worth noting that many Altmer (High Elves) resent Tiber Septim for his actions as a mortal. When Tiber Septim united Tamriel under one Empire, he conquered previously elf-held regions. The Thalmor's long-term goal involves unseating the Empire he originally established to regain control over Tamriel - a factor that could very well lead to the faction being the main villain of The Elder Scrolls 6. It makes sense, then, that the Altmer would reject the supposed divinity of their previous conqueror. More than that, however, Elder Scrolls lore generally suggests only elven races were able to ascend to godhood throughout most of Tamriel's history. The notion of a human being able to achieve the same could be viewed by the Altmer as a corruption of their religion, which only acknowledges the original Eight Divines.
Demanding humans reject what is arguably their most important divine figure (Talos is often called the hero-god of mankind, especially among the Nords of Skyrim) also helps exert control over the races the Thalmor seek to rule. It's strongly implied that the ban on Talos worship was expressly done in order to divide the Empire, weakening its overall strength to allow the Thalmor to more easily conquer it, should the war eventually resume. It could further be argued that the Thalmor are even actively seeking a Stormcloak victory in the Skyrim Civil War to weaken the Empire's hold over the region, thereby diminishing its overall presence in Tamriel. It's probable this issue will be discussed further in The Elder Scrolls 6, particularly if the game aims to continue the story threads introduced during Skyrim.
