Aristotle’s Organon was the essential logic textbook for most Christian educational institutions in the 16th century. Most editions of the Organon included Porphyry’s summary of Aristotle’s logic, the Isagoge. Zacharinus Ursinus, the author of the Heidelberg Catechism, produced his own version of the Organon along with Porphyry’s introduction to be used for teaching the basics of logic to youth. You can see his version here. For those who might want an introduction to Aristotle’s logic for the sake of understanding Reformed theology, Porphyry’s introduction is a good place to start. There is a new modern translation by Jonathan Barnes, one of the more preeminent Aristotle scholars. You can download a free edition here. I recommend that you get it while you can.