The construction of the “Chapelle Royale” was entrusted to the architect Jean Faulte on May 1, 1760. Its plan derives from that of the chapel of the Palace of Versailles.
Inside, the aisles surmounted by galleries present a superposition of columns with Ionic and Corinthian capitals, in the great classical tradition of French architecture. Nevertheless, the whole is treated with a freedom specific to the Louis XV style with certain details which announce the Louis XVI style. In October 1804, Napoleon signed a decree which granted the chapel to Protestant worship.
King Leopold I, who belonged to a branch of the Saxon family, was among the first to adopt the Reformation. He could not deny the faith of his ancestors; he therefore made this church the “Royal Chapel”. It is here that, once or twice a month, the sovereign attended worship, as before him, the princes of the Orange family. Charlotte Brontë, too, frequented the benches of the Royal Chapel while she was staying in Brussels in 1842. Her school, located rue Isabelle, was very close.
Here you can find a link to an interesting article (in French) about the church: https://www.bruxellesmabelle.net/lieux/chapelle-royale/