Complex sentence: The vast majority of its population was ethnic Javanese, consisting of more than 15,000 native Catholics, as well as a similar number of European Catholics; the number of native Catholics quickly outpaced the number of European ones, and by 1942 had doubled.
Split into simpler sentences: The vast majority of its population was ethnic Javanese, consisting of more than 15,000 native Catholics, as well as a similar number of European Catholics. The number of native Catholics quickly outpaced the number of European ones, and by 1942 had doubled.