The present edifice was built from 1330 to 1350 under Gerlach, Count of Nassau, as a collegiate church for a college of six canons, founded in 1333.

[1] Arnold Harnisch (Mainz) and Hans Martin Sattler (Idstein) removed the vaults and built Marmorarkaden (marble arcades).

[1] A unique feature of the church are 38 oil paintings, which completely cover the ceiling of the nave and the upper part of the walls.

The paintings, exclusively on biblical topics, were created from 1673 to 1678 by Michael Angelo Immenraedt from Antwerp and his assistant Johannes Melchior Bencard.

For example, The Wedding at Cana[2] on the south wall shows similarity to Rubens's painting The Feast of Herod, which hangs today in the Scottish National Gallery in Edinburgh.

[5] The topics of the paintings in the center row of the ceiling are, from the altar to the back: Verklärung Christi am Tabor (Transfiguration), Kreuzaufrichtung (Elevation of the cross), Auferstehung (Resurrection), Kreuzabnahme (Descent from the Cross), Himmelfahrt (Ascension), and Johannes auf Patmos sieht den Himmel offen und die Engel mit dem Evangelium (Vision of St. John on Patmos, literally: John on Patmos sees heaven open and the angels with the Gospel).

[1] Count Johann, infamous for his persecution of witches (Hexenverfolgung) as late as 1676,[6] died shortly before the reconstruction of the church was completed.

Franz Matthias Hiernle [de] constructed an epitaph for Georg August Samuel von Nassau-Idstein, his wife Henriette Dorothea and their children, who are buried there.

George Frideric Handel set the same biblical passage to music as Worthy is the Lamb, to conclude his oratorio Messiah.

Regular ökumenische Gottesdienste (ecumenical services) are held on the first Sunday in Advent (St. Martin) and Pentecost Monday (Unionskirche).

In 2017, the parish was awarded the Hessischer Denkmalschutzpreis (Hessian monument preservation prize) from the Landesamt für Denkmalpflege.

[12] The Landesamt für Denkmalpflege in Hessen, which cares for historic monuments in the state of Hesse, notes about the interior of the Unionskirche that firstly it follows the principles of a Protestant concept of church design, as they were first exemplified in the Hofkirche zu Torgau in 1544,[1] with Martin Luther's approval.

It is the first and heralding independent church-based creative structure ("erste und bedeutende eigenständige kirchliche Bauschöpfung") in Nassau after the Thirty Years' War.

[1] The organ, built in 1783 by Stumm [de], was replaced in 1912 by an instrument from Walcker, but retaining the historic case (Prospekt).

[18] In addition to the standard choral repertoire, Koch has selected rarely performed works such as Schumann's Missa sacra on 9 November 2008.

[17] In 2017, the first concert after restoration was dedicated to contemporary music including Ola Gjeilo's Sunrise Mass and Eric Whitacre's Five Hebrew Love Songs.