Because of I know the "official" history of those who "believe in the Pentecost" like Jack Twist's mother did and the fact that Roberta Maxwell, who portrayed her in the movie, said that Mrs. John C. Twist, Sr. was Pentecostal, I chose an alternate answer which was "
Non-organized Christian."
Pentecostal churches, independent congregations, semi-non-denomination and denominational ones, are not historically speaking Protestant churches.
According to Catholic and Protestant denominational histories, for a church to be officially a Protestant Church, it has to have split from the Roman Catholic Church AND have Martin Luther in its official history.
None of the Pentecostal Churches, referred to either as denominations or fellowships which began 100 years ago, are a split from any particular denomination, not the RCC nor any Protestant Church.
The United Pentecostal Church, an non-trinitarian doctrine denomination, does have in its history people who split from the Assemblies of God Fellowship which is Trinitarian in doctrine.
This year is the 100th anniversary of the Azusa Street revivals which took place in Los Angeles.
You can read some about it here at this link.
http://www.pbs.org/wnet/religionandethics/week935/cover.htmlThe offical website is here:
http://www.azusastreet100.net/The General Council of the Assemblies of God (AG) which has the Azusa Street meetings in its history can be found here.
http://ag.org/top/Annie Proulx does not have Jack Twist even discuss "The Pentecost" in her original story. Unlike what is said in the movie, and using my own experiences here and if he were an actual person, Jack Twist more than likely told Ennis Del Mar what the Pentecost was if his mother was actually Pentecostal, even if his father did not attend church regularly.
Since the movie's Ennis Del Mar stated that his "folks were Methodist," I believe that Ennis might have known a little bit about what "the Pentecost" meant, too; because the Methodists, like most Protestant denominations, have a "Pentecost Sunday" every year.
In the book, what is in the following quote is the only thing connected with any kind of religious belief.
Ennis rode easy, sleeping with his eyes open, but the hours he was away from the sheep stretched out and out. Jack pulled a squalling burr out of the harmonica, flattened a little from a fall off the skittish bay mare, and Ennis had a good raspy voice; a few nights they mangled their way through some songs. Ennis knew the salty words to "Strawberry Roan." Jack tried a Carl Perkins song, bawling "what I say-ay-ay," but he favored a sad hymn, "Water-Walking Jesus," learned from his mother who believed in the Pentecost, that he sang at dirge slowness, setting off distant coyote yips.
I know that I made this a little long here. Although I am Pentecostal by experience and my basic doctrinal beliefs are very much like the Assemblies of God's "16 Fundamental Truths," and I grew up attending regular church services and special services, I prefer to say that "I was raised AT home and not in a church." The only people that I have known who were raised in a church were those who actually lived in one while growing up. My Parents were Pentecostal by experience, too. But, they did not demand that their children believe just like them. They taught us to make our own choices and believe according to what we felt the Holy Spirit wanted us to believe.