Did Tertullian convert to Montanism?
I would say that Tertullian had a theological “flirtation” with Montanism. He believed in Montanism and its moral standards (stricter than ours):
Tertullian, undoubtedly the best-known defender of the New Prophecy, believed that the claims of Montanus were genuine beginning c. 207. He believed in the validity of the New Prophecy and admired the movement’s discipline and ascetic standards.
Tertullian did not convert to Montanism, but he maintained a view of its validity. By the way, the Church didn’t excommunicate Montanists en masse, just their various groups and it eventually died out. Tertullian stayed Catholic:
He believed in the validity of the New Prophecy and admired the movement’s discipline and ascetic standards. A common misconception is that Tertullian decisively left the orthodox church and joined a separate Montanist sect; in fact, he remained an early-catholic trinitarian Christian
I would think that belief (on the same level as a creed although perhaps not formalized as such) is not mere “flirtation”.
The Catholic Encyclopedia, s.v. “Tertullian” states:
It was after the year 206 that he joined the Montanist sect, and he seems to have definitively separated from the Church about 211 (Harnack) or 213 (Monceaux). After writing more virulently against the Church than even against heathen and persecutors, he separated from the Montanists and founded a sect of his own. The remnant of the Tertullianists was reconciled to the Church by St. Augustine.
The article was written in 1912, so there may be more scholarship since then.
Yup, Tertullian joined the Montanists (albeit some strange Donatist-ish, strict version of it).
That’s why he’s not “St. Tertullian.”
To be fair, he always had some Donatist-type issues (read “De Spectaculis” (On Shows), and you’ll see what I mean). It’s also speculated that his wife may have died at some point, because he gets a lot less mellow from that point on.