A branch of christians which protested against the estabilished church.

First of all it is necessary to contextualize the established church.

At the time of Jesus the " church" was the sanhedrin which were waiting a king and their cultural beliefs were more religious that other thing.

If this the context the people that didn't follow their (Sanhedrin) rules were considered as protestants.

But after Jesus time the any denomination which protest against the established. Originally the established church was the catholic church.

The protestant movement (Reform around 1500) is a big branch of christians which protested against the established chuch.