Answer
The crux of Jan Hus' grievances with the Catholic Church lay in his staunch opposition to various theological tenets and practices. He objected to the church's stance on ecclesiastical hierarchy and authority, denounced the selling of church offices (simony), and held reservations about the sacrament of the Eucharist, among other doctrinal differences. In the eyes of Hus, these perceived deviations from true Christian principles merited reform.