.
"The ceremony may have been in the Berlin cathedral, I'm not sure."
No. The civil ceremony tookplace at the Elector's Room at the Berlin Schloss, and then the bridal party made its way to the royal chapel where Pastor Ernst Dryander, royal chaplain, performed the wedding. He had also baptised Viktoria Luise and confirmed her as well.
The Prussians considered themselves Lutheran, Wilhelm made many references to his Lutheranism. In John Rohl's second volume of his bio on KWII, Rohl writes about how furious was WII at Ella's conversion. And Wilhelm "shared the opinion of Grand Duchess Maria Pavlovna, the Mecklenburg-born wife of Grand Duke Vladimir, that it was a "disgrave for a German Protestant princess to go over to the Orthodox faith in Russia at a time when both the Lutheran Church and her compatriots there were being persecuted and opressed in the most ruthless and injust manner."
Marie Pavlovna added that "the great, powerful German Reich and it's Kaiser" were now "the stronghold and the refuge" of the Lutheran church.
This is actually rather amusing, MP's comment as in 1908, she converted to the Orthodox church.
Pastor von Dryander was Lutheran. He died in 1922. In September 1918, KWII conferred on him the Order of the Black Eagle, and he became von Dryander.