Complex sentence: A Catholic convert, he was a protégé of Paul Xu (born Xu Guangqi) and also advocated repelling the Manchu invasion by modernizing Chinese weaponry and wrote treatises on geometry and military science influenced by the Jesuits' European knowledge.
Split into simpler sentences: A Catholic convert, he was a protégé of Paul Xu (born Xu Guangqi). Like his mentor, he advocated repelling the Manchu invasion by modernizing Chinese weaponry and wrote treatises on geometry and military science influenced by the Jesuits' European knowledge.