"非教会派的新教徒" can be translated as "non-conformist Protestants" or "dissenting Protestants".
"Non-conformist" emphasizes that they do not conform to the established or traditional practices of the main church bodies.
"Dissenting" conveys the idea that they hold differing or opposing views from the mainstream or established churches.
Both terms are commonly used to describe Protestants who are not affiliated with the main or established denominations.