Ultimately Joachim's lettrism is a product of his intense meditation on the Latin Vulgate, and he paid particular attention to numbers and correspondences throughout the text (everything is an echo of something), eventually weaving it all into one giant concordance between the Old and New Testaments

Joachim was introduced to the Tetragrammaton via Petrus Alfonsi, whose source was a 'Secret of Secrets' (not THE Secret of Secrets, just a compendium of esoteric material, part of which contained stuff from the Sepher Yetzirah).

An unrelated, but much earlier Christian lettrism (possibly older than the Sepher Yetzirah) is the 'Mystery of the Greek Letters' (6th c.) but it's about Greek not Latin. That said, even Joachim's stuff is, for example, a meditation on the Alpha and the Omega (Greek), and the Tetragrammaton (Hebrew)

Sorry I'm late replying to this. It's Joachim of Fiore.