The Old Calendarists do not worship the Old Calendar, they just want to be able to celebrate the feast days that have for many centuries been celebrated on a particular date. They do not like the Gregorian schismatic calendar as it is from the Vatican. By using that Roman Catholic calendar, the True Orthodox believe that the New Calendarists have caved into Ecumenism and papal infallibility.
As one usually finds in the arguments raised against the so-called New Calendar by the Orthodox, Maria refers to it as the 'schismatic calendar as it is from the Vatican' and later as 'Roman Catholic Calendar' presumbably because it was developed by scientists under the patronage of Pope Gregory.
Of all of the arguments raised by Old Calendarists, this is probably the most specious and one which opens them to ridicule.
As Wiki notes the relationship between the Catholic Church and science has been at times 'rocky' but "Originally most (scientific) research took place in Roman Catholic universities that were staffed by members of religious orders who had the education and means to conduct scientific investigation. Catholic universities, scholars and many priests including Nicolaus Copernicus (16th century Polish priest who determined that the sun was not the center of the universe), Roger Bacon (13th century Franciscan who is credited with the development of the scientific method), Albertus Magnus, Robert Grosseteste (13th century Bishop and founder of the scientific studies at Oxford) , Nicholas Steno (17th century bishop and pioneer in anatomy and geology) , Francesco Grimaldi (17th century Jesuit priest, physicist and mathematician), Giambattista Riccioli, Roger Boscovich (18th century Croatian priest and physicist, credited with developing the first atomic theories), Athanasius Kircher, Gregor Mendel,(priest and the 'father' of modern genetics), Georges Lemaître and many others, were responsible for many important scientific discoveries." http://en.wikipedia.org/wiki/Catholic_Church_and_science
Should we reject the work of say Copernicus or Gregor Mendel because they were 'schismatics' and under the 'control' of the Vatican?
Likewise in modern times the Catholic Church has been the sponsor of thousands of hospitals across the planet and funded hundreds of research labs at Catholic Universities where many important medical advances have been developed.
Applying the Old Calendar 'logic' to these works, are the Genuine Orthodox to disavow the subsequent fruits of these disciplines and the medical treatments developed under Church auspices? After all, by that logic they are 'schismatic' findings.
As Jah points out, there are plenty of good, ecclesiological arguments to be made against the adoption of the Gregorian Calendar as it has clearly impacted the unity of Orthodox expression worldwide, but the charge that the calendar is 'schismatic' is intellectually and honestly ridiculous.
Seriously, it isn't much of a leap from that point of view to the don't use Jewish doctors mindset.