Was defining the New Testament originally condemned as heresy?

It is impossible to know for sure without asking the person that said it, but I believe it is most likely, given her description of the individual in question, the historical time frame and the context of the discussion, that she was speaking of a man named Marcion.

Who was Marcion?

For the first hundred years or so of the Christian faith, documents like the letters written by Paul, and the four histories of Jesus, which we call “the Gospels”, had been circulated and copied throughout the churches for use in teaching. Very quickly, these documents came to be regarded as holy Scripture, of equal importance as the Jewish scriptures, which are referred to today as the Old Testament.

Around 140 A.D. (110 years after Jesus' death), a man called Marcion began teaching a version of Christianity which viewed the God of the Old Testament as a wrathful God incompatible with the loving God of the New Testament. He therefore rejected the theology of the Old Testament. To support his teachings, Marcion published a canon – a list of documents which he considered to be holy Scripture – which included only an edited version of the Gospel of Luke, and 10 of Paul's letters. This situation highlighted the need for consistent understanding in the Church of what documents (or books) were indeed canonical (i.e. accepted as authoritative), and could be used for setting and defending church doctrine. 1

It is a bit anachronistic for the above article and the video’s quote to refer to the “New Testament” since there wasn’t anything by that name at that time. Rather, for Marcion, his “New Testament” was not new in the chronological sense, but was new for the purpose of replacing the Old Testament.

Why was Marcion a heretic?

Marcion was not a heretic for writing down, or copying, the gospels and letters. He was not wrong, though perhaps a bit unorthodox, to make a list of books he personally considered authoritative. He was a heretic because he denied many of the major pillars of the faith such as the physical incarnation of Christ and thereby His physical death and resurrection, the authority of the Torah and that the God of the Jews is the one supreme Creator.

Unfortunately for Marcion, much of the established books of his day had at least some parts which disagreed with his beliefs. So Marcion decided to pick a canon of books, and then edit them to suit his theology. 2 It was this “writing down” of a “New Testament” that I believe the video is referring to, typically known as "Marcion's Canon."

Ultimately, the church excommunicated Marcion and produced apologetics, such as Tertullian and Irenaeus, against Marcionism. Our New Testament today was arrived at through a process of which Marcion was a quickening influence.

Is the documentary referring to Marcion?

Father James Bernstein, an Orthodox church leader wrote: "The first person on record who tried to establish a New Testament canon was the second-century heretic, Marcion. ... Many scholars believe that it was partly in reaction to this distorted canon of Marcion that the early Church determined to create a clearly defined canon of its own." (Which Came First: The Church or the New Testament?, Fr. James Bernstein, Orthodox churchman, 1994, p 7)

This statement, combined with Marcion’s heresy and excommunication, is most likely what was being referred to in the video. However, while the quote is technically true, Marcion really was just the first to claim a definitive canon. All the books he used, plus more, were already established as authoritative as seen by their use by church fathers. The fact he had an assortment of books from which to pick by name also shows that the core of the four gospels and the Pauline letters were already known and largely accepted.

Is the Documentary's Assertion Correct?

The Christian theologian who came up with the idea of having a New Testament was roundly condemned as a heretic.

In the end, this statement is, at best, misleading, if not a deliberate twisting of the truth. His condemnation as a heretic was not because of his canon. Rather, Marcion's canon was condemned because it removed books that were already accepted and removed the parts it disagreed with even in the few books he kept.

But yes, he did have the idea of a Canon, and yes, he was a heretic. But to say the one caused the latter is false.

1 - Theopedia - "Development of the canon"

2 – “Marcion and the Marcionites”