The firmness of R. lsserles finds its fullest expression in his commentary on the Shulchan Aruch and in the ד”מ, which served as a basis for the commentary. There we see R. Isserles as the authority who, in brief, clear and unequivocal terms, lays down the law and puts a final seal on it. “No one may deviate from it”, “this is the law” and many similar expressions add an air of finality to his decisions. Although, in the prefaces to the ד”מ on אור”ח and the תורת חטאת, R. Isserles reiterates his position that it is not his intention to force his views on others and that everyone is at liberty to decide as he sees fit, his popularity and air of certainty in the “Mapah” which is spread over the Shulchan Aruch, as well as in the תו”ח, had a most decisive influence on the spread and acceptance of his decisions among his people of Ashkenazic descent.
Rabbi Asher Siev, “The Period, Life and Work or Rabbi Moses Isserles” (PhD diss., Yeshiva University, 1943), 60-61.