system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k/30 Years War] What would have happened if the Chaos Gods had emerged earlier in human history, in the midst of the Reformation.

In the midst of the disastrous and fragmentary Thirty Years War the Protestant League fights against the Catholic Emperor and his allies. Unbeknownst to the warring powers, in the darkest recesses of the Warp the dark Gods of Chaos have awake millenia earlier than expected. In their wake comes the Damnation of Mankind.

Innumerable sects of Christianity flourish throughout Europe, and in their wake come the cults of Chaos. The newly awaken Gods feed on the energies of this bloody European war: Khorne flourishes in the endless death and destruction of the Thirty Years War. Tzeentch is powered by the now-constant desire by Christians to reform and change their Holy Mother Church. And finally Nurgle offers the ever-sought eternal life for the one bargaining chip all Christians have: their very souls. 

So, how does this play out?
gpt
This uh, was actually how it used to happen.  The Medieval Ages, and time after, were when the Gods of Chaos woke up in Lore, and still do.  They, instead of dealing with Humanity, dealt with the greater Galactic scene, because there was more war, change, and life out there than Human lands.

But if you must know, the Emperor would reveal himself at this point, most likely rally the Catholic peoples and their relators, and fight Chaos directly.  Khorne would feed the greatest but not eternally.  Aided by the great Gustavus Adolphus of Sweden, and the locals, driving Chaos from the Terran lands would be, truly, child's play.  The faith was strongest at this point, and would only grow stronger, become more ingrained in Human thought.

Think of the time - the Inquisition was around.  Teutonic Orders existed and aided.  Jerusalem was under Ottoman control, Christians angered by this.  A Christian War was raging, it was the perfect time for a common enemy to arrive and threaten all.  Without the advanced technologies of the 41st Millennia, Chaos would be severely limited to Human Technology.

Truly, it would be a great battle that would begin the Emperor's work earlier.

**EDIT: You're asking for Warhammer Fantasy, actually.  This is literally Warhammer Fantasy.**