Since the Reformation, the inhabitants of the Nassau-Ottoman principalities north of the Lahn belonged to the Reformed confession, those of the Nassau-Walram principalities south of the Lahn to the Lutheran confession, while the Principality of Nassau-Hadamar had been re-Catholicized since 1630. In the course of the secularization of ecclesiastical principalities in 1802/3 and the mediatization of smaller secular dominions in 1806, the number of Catholic subjects in Nassau increased considerably. In 1806, 44.8% of the approximately 300,000 inhabitants of the duchy professed the Catholic faith, 27.3% the Lutheran faith and 26.1% the Reformed faith. The remaining 1.8% included around 5,000 Jews and 170 Mennonites. After 1806, the aim was to promote state unity through the internal expansion of the state and to reorganize church relations with the aim of establishing a common state church.

The 300th anniversary of the Reformation in 1817 gave particular impetus to the unification efforts. Government President Karl Friedrich Justus Emil von Ibell, who promoted the project together with Duke Wilhelm zu Nassau, was able to convey the impression that the initiative did not come from the state, but from the churches and their official representatives. At a general synod in Idstein in August 1817, Lutheran and Reformed synod members, chaired by Ibell, negotiated the formation of a Protestant church comprising both denominations. Dogmatic and liturgical points of disagreement were excluded as far as possible. On the question of confession, agreement was reached on the Apostles' Creed and the Confessio Augustana, and the Electoral Palatinate Church Order was to be used in church services. Communion was to be celebrated in both forms and according to the rite of breaking bread. The first common catechism was not published until 1831, and it was not until 1843 that the Union Church of Nassau received its own liturgical order. On 11.08.1817, Duke Wilhelm recognized the ldstein resolutions. The ecclesiastical and organizational details were regulated by edict dated 08.08.1818.

The Nassau Union is at the forefront of all comparable processes in other German states. The Union of the Protestant Churches of Prussia of 27.09.1817 came closest in time. Like the founding act, the practical implementation was harmonious and without any significant friction. In 1917, on the occasion of the 100th anniversary of the Nassau Union, the Idstein church was given the name "Union Church".