Llull's Art was a serious and strange project. Beginning after a religious conversion in his thirties, and refined across several decades and several versions, it reduced knowledge to a set of basic principles, goodness, greatness, duration, power and the rest, and combined them mechanically through tables and concentric rotating discs, so that the truths of theology could in principle be generated and demonstrated by manipulation rather than assented to on authority. The purpose was missionary: he wanted a method that would compel the assent of Muslims and Jews without appeal to scripture they did not accept, and he learned Arabic for the same reason.
It did not convert anyone, and the modern interest in it is different from his. The Art is a genuine ancestor of combinatorial logic and of the project of a universal formal language; Leibniz read Llull as a young man and his De arte combinatoria acknowledges the debt directly.
One thing needs stating flatly because the tradition has it wrong for centuries. The alchemical works attributed to Llull are not his. A substantial pseudo-Lullian corpus was composed after his death and circulated under his name, and it is the reason he appears in histories of alchemy and in occult reference books as an adept and a transmuter. The historical Llull wrote against alchemy. The attribution is one of the largest and longest-running misattributions in the field, and it is settled.