Pentecostalism's beginnings are rooted in the description of "tongues of fire" that fell upon the heads of Jesus' followers who gathered to pray in Jerusalem around the time of the Jewish Festival of Weeks (Pentecost). The Book of Acts (2:4) continues, "All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them." The Book of Acts, along with Paul's commentary and instructions regarding the other nine gifts of the Holy Spirit, form the central tenets of Pentecostalism.
From this description, what Pentecostalism is, where it began, and how or if it still continues into the present are all issues up for theological and historical debate. What can be described as Pentecostal beginnings might better be framed as "charismatic" activity, which was attested to hundreds and hundreds of times over the span of the formative years of the Church. This is important to note, since one of contemporary Pentecostalism's key arguments for its validity as a core part of Christian history is its existence as part of an unbroken stream of supernatural activity spurred by the direct experience of the Holy Spirit, thus tying all Pentecostals back to the Book of Acts.
Most of this Pentecostal activity in the early Church followed familiar biblical patterns focusing on prophecy, visions, healing, and exorcisms (casting out of demons). Specific descriptions of speaking in tongues (glossolalia) are often nuanced, inferred, but there are some descriptions of speaking and singing in languages that were not known to the speakers (xenoglossia). Since glossolalia and xenoglossia are both a part of Pentecostalism's contentious history, it should be noted that both phenomena were described in early Church circles. Suffice to say, historically there have been detractors who have doubted that either of these phenomena is possible, and they often posited that Church leaders who had experienced such phenomena were either deluded or deceived, and that most were theologically very dangerous.
Montanus, a 2nd-century Church leader, was considered a heretic because of his claims to receive direct revelations from the Holy Spirit. Montanus, and his female companions, Priscilla and Maxmilia, preached throughout Phrygia (modern-day Turkey), that direct revelations from the Holy Spirit had given them the ability to prophesy and to receive visions, and a renewed zeal for prayer and fasting. Challenging the notion that one did not need the authority of the Church to be a faithful Christian, Montanus and other leaders who believed that the Holy Spirit could lead them to the truth without the guidance of the Church's authority were simply too dangerous for the Church to accept. Most, if not all, of Montanus's followers were branded as heretics, including noted Church leader Tertullian.
Montanus is probably one of the more well-known of the early Church proponents of direct experience with the Holy Spirit, but not all such would be labeled as heretics. Church leaders who claimed to have had some charismatic experience included noted heretic-hunter Irenaeus (c. 115-202), Origen (c. 185-254), Augustine (354-430), Symeon (Eastern Orthodox) (949-1022), and even Thomas Aquinas (1225-1274); they are joined by a host of early and medieval mystics of the 15th and 16th centuries.
Many of these figures would describe phenomena that contemporary Pentecostals would identify as Pentecostal gifts of the Spirit, or at least, Pentecostal-like phenomena. Irenaeus was believed to have had the gift of prophecy, discernment of spirits, and exorcism. Origen was reported to have healed people, exorcised demons, and engaged in other assorted "signs and wonders." Some mystics, including Eastern Orthodox figures Symeon the New Theologian and Seraphim of Sarov, discussed phenomena such as "baptism of the Holy Spirit," uncontrollable bouts of crying, and visions of the Transfiguration (akin to the description in Matthew 17) overtaking them for hours on end.
Some significant Church leaders did describe what Pentecostals today would believe is glossolalia. Francis Xavier (1506-1552), Vincent Ferrer (1350-1419), Ignatius of Loyola (1491-1556), and Teresa of Avila (1515-1582), among others, describe something akin to Ignatius's description of prayer: "During the interior and exterior loquela (speech) everything moves me to divine love and to the gifts of loquela divinely bestowed." (Spiritual Diary, #222).
Despite the controversy that followed such experiences (nearly all who admitted having these experiences risked attracting the wrath of Church officials charged with rooting out all suspected challenges to ecclesiastical authority), some of the most well-known Christian figures have reported these experiences, made these practices a part of their theological legacies, and often paid a heavy price for insisting that they had direct access to revelation through the Holy Spirit. Throughout history, those who claimed these experiences were often targeted as heretics, threatened with persecution (such as the Inquisition), and, if they posed enough risk to Church authority, were removed from any mantle of leadership. Thus, groups as varied as the Spanish "alumbrados" (Enlightened Ones), the Jansenists, or a legion of Christian mystics, have been made to answer for taking what was supposed to be the domain of the Church (access to revelation), and making it available to anyone who asked for it.
By the mid 1700s, early forms of contemporary Pentecostalism began to emerge from what began as a reformation movement within Anglicanism; they resulted in a global movement that has influenced nearly every different tradition of Christianity.