It's an interesting use of the English & Welsh criminal court system (can't speak for the Scottish system) which has largely fallen into disuse. An aggrieved party alleging a criminal offence presents his case to a magistrate. The magistrate - in his role as Examining Justice - determines whether there is prima facie evidence of an offence, can issue summonses and directs the investigation (this is the French/Continental model) and can issue arrest warrants.
Nowadays in E&W the practice is usually different but it harks back to this system - all citizens are granted powers of arrest without warrant but 'constables' have powers of arrest in wider circumstances (basically 'on suspicion').
This is a District Judge who has issued the summons. DJs are qualified lawyers (most magistrates are lay people) and Westminster MC is not known for allowing frivolous cases.
The difficulty - as noted above - is following the case through. If the summons is ignored a warrant for arrest will be issued. That brings the extradition issue into play - and the US is notoriously not keen to extradite its citizens. But an EU arrest warrant could be issued which might lead to the 'interesting' situation where the Head Honcho (aka Inspired Chief Prophet or whatever) cannot travel anywhere in the EU for fear of arrest. Will their god protect him, or is this an indication of the Persecution all cults thrive on?
Whatever, anything that brings ridiculous beliefs and practices to public notice is a Good Thing, IMHO.