Fairly valid, if you buy into the prejudices and definitions on which Voltaire was clearly basing his statement. The first part is easy--Voltaire was not a fan of state-sponsored religion, and certainly the long history of the HRE produced plenty of gross princely misbehavior to argue against the application of the term. The actual term in its' original Latin, however, is sacrum, i.e. 'consecrated,' and relates to Frederick I's ambitions vis-a-vis the Papacy. The term 'Roman' in this context, is obviously false in a strict sense--the HRE was an alliance of mostly Germanic states, but it did have a (rather messy) descent from the late, 'barbarian' emperors of Rome. The 'Empire' bit is similarly confused. The HRE was a confederation of principalities who elected an Emperor but enjoyed almost total latitude in internal affairs. It bore little resemblance to other 'empires,' although, to further confuse the matter, the Emperor also had direct control over some lands himself.