On 28 September 1883 he was appointed Bishop of Luxembourg,[1] and was consecrated on 4 November of the same year by Cardinal Edward Henry Howard.

[1] At the beginning of his tenure as bishop, he officially recognised the controversial Dominican community of the monastery on Limpertsberg around the stigmatised Anna Moes (1832–1895).

[1] This helped ensure that Catholicism would become the leading cultural as well as political force in society after the First World War.

[1] His time in office also saw fierce and bitter public disputes, amongst other things over the controversial education law of 1912 and over press issues, in which Koppes took an uncompromising stance, and which damaged church-state relations (which had never been good in the 19th century).

Instead, he was buried (like his predecessor Nicolas Adames) in the Glacis chapel in front of the walls of the former Fortress of Luxembourg.