Such is the claim of Baptist preacher Dr. James Milton Carroll, the author of the "Trail of Blood" pamphlet. He says that the Baptist church was not just a creation of the 17th century, but was present before then:
Donatists -- Otherwise Catholic rigorists who believed that the Sacraments could not be effected by priests who had apostatized during the persecutions, that is that the Sacraments are dependent on the personal sanctity of the priest.
Paterines -- Catholic reformers in Milan who sought to cleanse the Church of simony and enforce clerical celibacy. St. Arialdo da Carimate was a leader of the movement and was tortured and murdered by fellow clerics.
Cathari -- A Gnostic sect. They had two levels of adherents -- the Believers and the Perfects. The Perfects were held to high standards of fasting, such that many Cathar Believers did not go through the ceremony until their death bed. Like the Marcionites and other Gnostics, they believed the world was not created by God but by the demiurge ("rex mundi"), who they associated with satan. They rejected water baptism.
Paulicans -- Followers of Paul of Samosata. They were Gnostics who also believed that the world was created by the demiurge and thus rejected the Old Testament. They condemned veneration of the Cross and were Adoptionists (they believed Jesus *became* God at some point, such as at His Baptism). The Bogomils were another Gnostic group that formed out of the Paulicans.
Anabaptists -- Part of the Radical Reformation, today in the form of Mennonites, Amish, and Brethren. They were called "ana-baptists" ("ana" = "again") because they re-baptized people into their sects, contrary to Scripture and the Creed (the same reason why Baptists are called such). Some anabaptist groups adopted non-Trinitarian theologies and other heresies, such as the Christadelphians, Michael Servetus, and the Socinians (followers of Faustus Socinus).
Petro-Brussians -- Followers of Peter of Bruys, who denied the authority of all books of the Bible except the Gospels. He rejected the Incarnation, accepting Docetism (the Gnostic heresy that Christ's Incarnation was an illusion). Because he denied the Incarnation, he was also violently opposed to crosses and crucifixes, so he made a bonfire of them outside of a Church on Good Friday. The locals were not pleased with his blasphemies and threw him on the flames as well.
Arnoldists -- Followers of Arnold of Brescia, an Augustinian prior. He taught radical poverty similar to the so-called "Spiritual Franciscans" (distinct from orthodox Franciscans) or Waldensians. That is, he taught that if priests, bishops or monks held any property, they could not be saved. He also echoed the Donatists who relied on the personal sanctity of the individual to give the office authority and to effect the Sacraments.
Henricians -- Followers of Henry of Lausanne, heavily influenced by Peter of Bruys, the Gnostic, except without the contempt of the Cross (he carried a staff with a iron cross on it). He rejected all doctrinal and disciplinary authority of the Church, rejected the communion of saints, and rejected any form of liturgy and worship, and embraced free interpretation of the Gospels.
Albigenses -- Another name for a later revival of "Catharism", a form of Gnosticism, named after a debate between Catholic priests and Cathars held in Albi (Albiga), France.
Waldenses -- Followers of Peter Waldo. Their practices follow that of the Cathars in their division into Believers and Perfects. However, theologically they were more orthodox. The primary issue was the pastoral failure of the Catholic Church, so the Waldensians organized themselves and created their own interpretations of the Scriptures (but nothing terribly theologically radical). In many ways they are similar to Protestants. It was for that reason that during the Reformation, they adopted Calvinism/Zwinglianism. Most blended with the Reformed churches but by name they still exist in the form of the small Waldensian Evangelical Church in Italy. Unlike the Baptists, the Wandensians have always practiced infant baptism.
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So what do we notice? For one, there is no obvious historical link between most of these different groups and some teach very different things. If these are supposed to represent pre-17th century Baptists, wouldn't you at least expect none of the groups to have practiced infant baptism (the rejection of which is the cause of the very name of the Baptist sect)? Secondly, several of the groups are outright Gnostics.
A 1991 survey by the Southern Baptist Convention found that 14% (1 in 7) of their pastors and 18% of their deacons (nearly 1 in 5) were Masons. It is estimated that 37% of Freemasons are Southern Baptists. Such prominent Baptist leaders and theologians such as B.H. Carroll (founder of Southwestern Baptist Theological Seminary; brother of the author of this pamphlet) and Robert E.B. Baylor (founder of Baylor University) were Freemasons. Freemasonry is Gnosticism, Scottish Rite Freemasonry even openly refers to Gnosticism several times in its rites.
So the Landmarkist Baptists, following Dr. Carroll, claim that the Baptist religion is founded on various Gnostic sects. The brother of the author was a Freemason, which is a Gnostic religion. (I do not know if there is evidence that the author was a Mason but I'm sure there was some influence here.) Today, many Baptists are also Freemasons, and thus Gnostics.
So the question must really be: How much of a connection is there between Baptists and Gnostics?