The Oxford Latin Course presents the numerous declensions of is, ille, hic, and ipse early in their introductory course (chapter 8 of 31).
I am wondering if I should stop here and memorize all of these declensions before proceeding. How proficiently can one read Latin without memorizing all of the demonstrative pronoun declensions? Or are they only needed for writing?
You can't read Latin proficiently without knowing these pronouns: they're all very common, and the meaning of a sentence often hinges on their precise form.
That said, their declensions are complicated and tend to be something students of Latin come back to again and again until they're mastered. So you don't necessarily need to stop here until you've memorized them perfectly, but definitely make a start on learning the forms if you want to eventually read Latin well.
(Btw, I think many textbooks make a pedagogical mistake in presenting these pronouns together: too many forms at once, with the result that lots of students kind of give up on them. It might be better to learn one at a time, but I realize you may not have that option as you're following the OLC.)
