"Take any book on homological algebra, and prove all the theorems without looking at the proofs given in that book. Homological algebra was invented by Eilenberg-MacLane. General category theory (i. e. the theory of arrow-theoretic results) is generally known as abstract nonsense (the terminology is due to Steenrod)."
— Serge Lang, Algebra, First edition: page 105; Second edition: page 175.
I have been using homological algebra in some of my research, so I am offering an introductory course on homological algebra during the spring semester of 2025. For details, please check this link.