Montanism is a heresy that was denounced by the 2nd-century Church after its founder, Montanus, began a movement that he referred to as “New Prophecy.” The movement largely consisted of the belief that the Holy Spirit granted direct, divine revelation which manifested itself in “ecstatic utterances.”
The adherents of the movement, which the Church anathematized, were seen largely by the orthodox believers as false prophets and apostles.
The Montanists believed themselves to be inspired by the Holy Spirit, therefore they believed that their utterances were equivalent with the revelations recorded in Scripture essentially denying the final authority of the Scripture.
Interestingly, we see a strikingly similar movement in the Church today in the charismatic movement. The charismatic movement also holds to the notion that individuals, under the inspiration of the Holy Spirit, can receive direct revelation from God apart from the Scriptures. Much of what we see in today’s charismatic movement is identical to that of the Montanists, including the ecstatic utterances (babbling, gibberish), direct revelation, and uncontrollable movements which appear to be more of along the lines of demon possession than anything from God.