The book is about two beings fighting back and forth called Preservation and Ruin but preservation ends up being called the mist and I also remember there being an old guy who studied numerous religions and stored all his magical power into plates he could tap into and release it.
Similar to FuzzyBoots' answer but this sounds specifically like Hero of Ages the third volume of Brandon Sanderson's Mistborn Trilogy (part of his Cosmere universe).
The guy who studies religions is Sazed and he is a Feruchemist, a being who can store powers or abilities in metals to use later. Metal plates do factor significantly in the story as it is revealed that
Ruin is unable to change anything written on metal.
Messengers with metal plates then become the only reliable means of getting messages across distances.
The history of Preservation and Ruin are explained in this book as well.
Is it possible you're thinking of Brandon Sanderson's Cosmere books?
The Mist is a gaseous manifestation of Preservation's power similar to both the liquid in the Well of Ascension and the beads of Lerasium. The Mist is far less dense than Preservation's liquid form, and as such is more limited in what can be done with it. Where Preservation's liquid form can be used to do great things, from moving planets to changing or creating living creatures, the Mist seems to only be able to function incidentally as fuel for allomancy.
The Mists were originally set up as a method to create Mistings by Leras via the mist spirit. This Spirit took people that fell well below the level needed for the physical or emotional pain to be able to snap them, and then both created a trauma while raising their level of preservation in their body. This process could be fatal to the person snapped depending on many factors one of which is what type of Allomancy the person is getting. Some have only a little trauma, and others have a debilitating trauma inflicted on them that requires weeks of bed rest. The elderly and young are more susceptible to death during this process. The Mist reacts to any Allomancer by swirling around them so long as they are not pierced by a Hemalurgic spike. Any Allomancer with a spike will slightly repulse the mists as if they were pushing it away from them. The Mist that obscures the night can be partially pierced by an Allomancer burning tin to reveal the stars above, a tin Savant can even pierce them to the point that they are virtually gone to their sight. The mists will not enter any building and even a simple tent is enough to keep them out. In special circumstances some Allomancers can and have drawn on the mists for power, using them as fuel for Allomancy even after their ingested metals have all been burned. This use of the mists allows for greater power when using Allomancy to the point that a person can even push on metals piercing someone's skin. Ruin's power is sometimes seen as a Black Mist where Preservation's is a White mist. This manifestation of Ruin's power is only seen at the Well of Ascension and After Ati's death before Sazed picked up both shards.
In particular, The Preservation is indeed opposed to Ruin, and is known as the mist spirit.
Preservation is the fundamental opposite of Ruin. He made a deal with Ruin to create humans, under the agreement that one day Ruin could destroy everything. He later betrayed Ruin to stop the destruction by giving most of his consciousness to imprison him and take away his body, which was discovered to be the Atium itself. This reduced him to the wisp of himself which is known in the books as the mist spirit. It is revealed that the mists are Preservation's body and power which is why they swirl around someone using Allomancy.
The Aon Dor were inscribed on various metal plates in the series.
Elantris(city) had metal plates inscribed with Aon Ashe for light. This thread is an expansion of that concept to other Aons, in an attempt to replicate the modern society on Earth, while on Sel. Aon applications are built around what I am calling an Aonic Circuit Board: a metal plate inscribed with Aons and modifiers. I have taken the liberty of assuming that by the time Sel reaches a tech level like Earth, they will either have the ability to do this with precision, or have found a way to make machines that can do this for them. I have also made the assumption that a method has been devised to let non-Elantrians affect Aons via a middle-man(example: a light switch)
He is described as being a tall muscular man with dark skin and pure white hair. He wore a golden costume of billowing trousers and a coat which came to his waist. As his shard was Honor it is likely that he was forced to base his actions on honour and obligations.
While not explicitly old, the while hair might suggest otherwise.
I'll admit that I've never read the series, but it was what popped up for a search of book preservation ruin "the mist"