This might be too vague or too broad if we're not careful. Therefore, let's focus on the basics. According to this MSE search, this is new to MSE.
Some Background:
I have read Goldblatt's, "Topoi: A Categorial Analysis of Logic," - although Chapter 14 is where I stopped doing its exercises altogether, which I had been gradually neglecting as I read the text. It covers some ground in its opening chapters.
In this comment on a question of mine, three books on category theory were recommended.
I'm reading Mac Lane and Moerdijk's, "Sheaves in Geometry and Logic: A First Introduction to Topos Theory." Its very first chapter is on categorical prerequisites and I have read it.
Around 2013, I read most (if not all) of Turi's "Category Theory Lecture Notes" from The University of Edinburgh. I've forgotten most of it though.
The Question:
What are the prerequisites for topos theory?
Context:
I am teaching myself topos theory because I find it fascinating and I enjoy the challenge.
I'm not sure where I get this notion from but I'm given to understand that algebraic geometry plays a rôle in topos theory. I did a module on algebraic geometry in the final year of my MMath.
The same goes for topology.
I have included the book-recommendation tag because, as above, some of you might feel it best that I read some other books before continuing with Mac Lane and Moerdijk's.
Please help :)