Pentecostalist is used of the same Christian tradition as Pentecostal: churches that stress the Holy Spirit’s power, including healing and, often, speaking in tongues. The extra -ist more often marks a person who belongs to that tradition, the way Baptist can mean a churchgoer, not only a style. People also call that person a Pentecostal, so the two words overlap a lot. Pentecostalist is a little less common, and a little more “the follower.” You can still hear Pentecostalist churches or Pentecostalist teaching, but Pentecostal is the usual adjective. Nothing in the extra ending changes the faith being named.