The ancient Essenes are known as being an highly Ascetic sect, with many Mystical and Monastical practices.
In fact, Rav Aryeh Kaplan seems to imply that much of our Kabala actually originated from the Essenes, who supposedly inherited them from the Nevi'im, and from Pythagorean teachings (which the Greeks themselves admit they got from the Ancient Egyptians, Hebrews, and Phoenicians).
But in the Artscroll commentary on the Amidah, they are labeled by Artscroll as heretics, with no explanation. So were the Essenes heretics, or just some highly Ascetic groups of people?