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| Joachim Stegmann the Elder. | 132 |
| Joachim Stegmann the Younger. | 162 |
| John Becius. | 164 |
| John Biddle. original: "Johannes Bidellius"; an influential English teacher often called the "Father of English Unitarianism." | 159 |
| John Campanus. | 17 |
| Jan Kazanowski (Johannes Cazanovius). | 49 |
| John Crell (Johannes Crellius). | 115 |
| Jan Falkonius (Johannes Falconius). | 54 |
| Johannes Geisteranus. | 113 |
| Johannes Glirius. A pseudonym for Matthias Vehe, a radical theologian. | 60 |
| Johannes Hartigveld. | 161 |
| Jan Kiszka of Ciechanowiec. A powerful Polish-Lithuanian magnate and patron of the Polish Brethren. | 82 |
| Jan Licinius (Johannes Licinius). | 94 |
| Johann Ludwig von Wolzogen (Johannes Ludovicus Wolzogenius). | 137 |
| Jan Niemojewski (Johannes Niemojovius). | 49 |
| Giovanni Paolo Alciati (Johannes Paulus Alciatus). | 27 |
| Johann Preuss (Johannes Preussius). | 163 |
| Johann Sommer (Johannes Somerus). | 57 |
| Jan Stoiński (Johannes Stoinius). | 121 |
| Johannes Sylvanus. | 60 |
| Juan de Valdés (Johannes Valdesius). | 2 |
| Giovanni Valentino Gentile (Johannes Valentinus Gentilis). | 26 |
| Johann Völkel (Johannes Volkelius). | 96 |
| Jonas Schlichting (Jonas Slichtingius). | 126 |
The Kiszka [Lady], Palatine of Vitebsk. original: "Kisciana Palatina Vitepscensis"; likely referring to Anna Kiszka, a noblewoman of the influential Kiszka family. 54
Lelio Sozzini (Lælius Socinus). The uncle of Faustus Socinus; his private papers provided the foundation for what would become Socinianism. 18
Lancelot van Brederode (Lancelottus à Brederode). 134
Wawrzyniec Krzyszkowski (Laurentius Criscovius). 54
Lawrence Stegmann (Laurentius Stegmannus), otherwise known as Tribander. 134
Ludwig Hätzer (Ludovicus Hetzerus). 16
Marcello Squarcialupi (Marcellus Squarcialupus). 81
Martin Borrhaus (Martinus Cellarius). 15
Marcin Krowicki (Martinus Crovicius). 45
Marcin Czechowic (Martinus Czechovicius). 50
Martin Ruar (Martinus Ruarus). 114
Matteo Gribaldi (Matthæus Gribaldus). 17
Matthäus Radecke (Matthæus Radecius). 84
Matthias Glirius. 60
Melchior Schaffer. 143
Michael Gittich the Venetian (Michaël Gittichius Venecianus). 108
Michael Servetus (Michaël Servetus). The Spanish physician and theologian whose execution in Geneva for heresy became a turning point in the history of religious tolerance. 6
Nathaniel Stuckey (Nathanaël Stuckey). 172
Mikołaj Kochanowski (Nicolaus Cochanovius). 83
Niccolò Paruta (Nicolaus Paruta). 25
Mikołaj Sienicki (Nicolaus Sienicius). 49
Petrus Geisteranus. 114
Peter of Goniądz (Petrus Gonesius). One of the earliest promoters of Anti-Trinitarianism in Poland. 40
Piotr Mączyński (Petrus Maczynski). 83