The real distinction between a thing’s essence and its existence is a key Thomistic metaphysical thesis, which I defend at length in Scholastic Metaphysics, at pp. 241-56. The thesis is crucial to Aquinas’s argument for God’s existence in De Ente et Essentia, which is the subject of an eagerly awaited forthcoming book by Gaven Kerr. (HT: Irish Thomist) One well-known argument for the distinction is that you can know thing’s essence without knowing whether or not it exists, in which case its existence must be distinct from its essence. (Again, see Scholastic Metaphysics for defense of this argument.) In his essay “How to Win Essence Back from Essentialists,” David Oderberg suggests that the argument can be run in the other direction as well: “[I]t is possible to know that a thing exists without knowing what kind of thing it is. (Such is our normal way of acquiring knowledge of the world.)” (p. 39)
(Transcript here.) An SNL skit illustrating a key theme of Thomistic metaphysics? Not so surprising given that Martin was a philosophy major and Murray is a fan of the Latin Mass.