Nicolaus Copernicus (1473 – 1543) was a Polish church canon who is most famous for writing De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres) a book that argued that the cosmos was better understood if the Sun was at the centre of the universe rather than the earth.
Although a churchman, Copernicus was never a priest, and was largely an administrator and diplomat, who had studied medicine and also acted as a physician when needed. His work as an astronomer would not have seemed out of place as astronomical observations were very important in organising the calendar which was a major concern of the Church.
The idea that the Earth moved through space and revolved around the Sun with the other planets was revolutionary, and also contradicted Catholic Church teaching. When De revolutionibus was eventually published in 1543 (the story is that Copernicus received one of the first copies on his deathbed) it included an anonymous preface which had been inserted by a colleague (Andreas Osiander) to the effect that the model descried by Copernicus should not be understood as intended to be a realistic account but rather should be seen as a hypothetical model which might prove useful (although Copernicus's model included many of the complications of the existing geocentric theory as he held to the assumption that heavenly bodies would move in perfect circles – so complex systems of circles within circles, or circles moving eccentrically, were still needed to fit observations).
The Church did not seek to censor or ban the book for some decades – although it was eventually put on the 'Index' of Prohibited books until some 'corrections' had been made by the censor. This change of attitude was likely due to the attention brought to the book by later champions of heliocentrism. In particular, Galileo Galilei's famous trial and conviction by the Church inquisition was based on the charge that in his teaching and writings he did not limit himself to presenting the Copernicus model as a theoretical notion. Galileo claimed at his trial that he had never actually held or taught the heliocentric account as being true, even if he had given that impression – but it is very hard to read some of his work as anything but a strong argument for the reality of the Earth's movement around the Sun!
While Copernicus's model was in many ways as unwieldy as the existing model due to Ptolemy that had been the conventional view for centuries, it provided the basis for Johannes Kepler's later work in which he (eventually, with some reluctance) replaced circles within circles within circles by elliptical orbits which much more readily fitted observations of planetary motion.