I make this chief distinction between religion and superstition, that the latter is founded on ignorance, the former on knowledge; this, I take it, is the reason why Christians are distinguished from the rest of the world, not by faith, nor by charity, nor by the other fruits of the Holy Spirit, but solely by their opinions, inasmuch as they defend their cause, like everyone else, by miracles, that is by ignorance, which is the source of all malice; thus they turn a faith, which may be true, into superstition.

Letter 21 (73) to Henry Oldenburg, November (1675)