You could call the adoption of historicism a paradigmal shift in historiography. It took place in the 19th century. Previous eras had looked to religion, philosophy or even the natural sciences to find timeless general truths and laws in history, but historicism meant understanding each society and era in terms of its own unique individuality. It rejects the existance of general laws in history - instead it recognized the human actor. Although it stresses the difference between nations and eras in historiography, historicism does not reject general histories.

Historicism became the norm during the 19th century. However, already in the 18th century we can spot men who have historicist tendencies in their work and have elaborated on the essence of the historicist idea. I've been taught Johann Gottfried von Herder (1744-1803) is the most important of them. He argued that there was no universal standard to measure societies (a popular idea at the time), with every country having unique characteristics, a unique essence. He emphasised national historicism. Even earlier, there's Giambattista Vico (1668-1744) and Adam Smith (1723-1790), who had a cyclic view of history (society going through different fixed phases of development), and this is often branded as temporal historicism, for they judged each of these phases on their own terms.

If we look at this discription of historicism and then at Ranke's ideas, we can certainly see that he endorses the new historicist paradigm. He advocates that historians do not judge the past according to their own contemporary standards, but instead objectively report what the primary sources say. Human actors are central in the form of the state (as Rankean history is very much political history). Ranke recognizes the uniqueness of different eras. Ergo, Ranke is indeed part of a bigger movement whose ideas had been developing since the 18th century. Him basically being the father of modern historiography, however, did mean that his adherence helped historicism become the norm.

Sources: J. Popkin, "From Herodotus to H-Net: the story of historiography", and a B.A. research seminar on historiography