What is the ruling on eating pizza that contains cheese mixed with pork fat, which was not known about until after eating a slice of the pizza?

It is forbidden to eat food mixed with lard (pork fat) due to its impurity, based on the Almighty's saying: ﴿Say, "I do not find in what has been revealed to me anything forbidden to eat for one who eats it, except that it be a dead animal or spilled blood or the flesh of swine — for indeed, that is impurity."﴾ Repentance is obligatory, and one must always verify the permissibility of foods.