Stillemans began studies at the St. Joseph Minor Seminary of Sint-Niklaas in 1846.

In 1853 he enrolled at the major seminary in Ghent, earning a doctoral degree in philosophy at the University of Leuven in 1860.

He became president of the Major seminary in 1888 and canon of the cathedral chapter of Ghent and member of the episcopal council.

In 1899, under pressure from the royal court and Minister Woeste, he suspended Adolf Daens, a radical priest in Aalst.

The colors of the city, blue and yellow are reflected in his heraldic episcopal crest.