The town of Siegen belonged in the beginning to the Archbishopric of Mainz, or more precisely to its deaconry of Arfeld. There was a White Nun convent in town that folded in the 15th century. Furthermore, there was a Franciscan Monastery that was dissolved in 1533 after the Nassau overlords had introduced the Reformation in 1530. After that, the town was first Lutheran, but in 1550, the Principality of Nassau converted to the Reformed Church. Subsequently, Siegen was a predominantly Protestant town, but not so strongly that the Counterreformation could not gain ground in 1623, with one fifth of the townsfolk and those living in the surrounding area becoming Catholic once again. As of 1626, there was once more a monastery in town, this time a Jesuit one. After passing to Prussia in 1815, the union between Lutheran and Reformed churches was introduced in Siegen, as it was throughout Prussia, but the town's parishes kept their Reformed emphasis. As part of the Westphalian Provincial Church (now the Church of Westphalia), Siegen became the seat of a Superintendency. A similar entity still exists in Siegen, known as the "Kirchenkreis", or church district, to which all the city's parishes nowadays belong, unless they are Free Church parishes. This church district encompasses the whole of South Westphalia all the way to Olpe.