The first source mentioning this theonym is the Gesta Hammaburgensis ecclesiae pontificum by Adam of Bremen: The elderly Bishop John, captured with other Christians in the city of Mecklenburg, was kept alive to be exhibited in triumph.
And consequently, lashed with whips for having confessed to Christ, he was then paraded in each of the cities of the Slavs to be mocked, as he could not be forced to renounce the name of Christ, his hands and feet were cut off and his body was thrown into the street, but not before removing his head, which the pagans stuck on a pike and offered to their god Radegast as proof of victory.
[2] Following Adam,[3] Radegast is also mentioned by Helmold in his Chronicle of the Slavs, who writes about making annual sacrifices to him and using an oracle associated with his temple,[4] he also calls him "the god of the Obodrites".
[5] It is also mentioned in the Annales Augustani of 1135, which tells of the destruction of Rethra by Burchard II, Bishop of Halberstadt, who took the local "horse worshipped as a god" on which he returned to Saxony.
[8] Today, the name Radegast is predominantly used in English,[9][10][11] but in several Slavic countries like Poland and Russia, the prevailing notation is Radogost.
Croatian: Radogost,[19] Old Polish Radogost, Radgost, Radogosta, Radosta,[27][28] Old Slovene Radegost,[19] probably attested as early as the 6th century in a Greek source mentioning a Slavic tribal chief named Ardagast (Ancient Greek: Αρδάγαστος; form before probable metathesis).
[30] Thietmar, in his Chronicle (written around 1018 r.[31]) states that Svarozhits (recognized as a solar deity[32]) was the most worshiped god in Polabian Radogošč.
The same town, however mentioned under the name of Rethra (Latin: Rethre), is also described about 50 years later by Adam of Bremen, who recognizes Redigast as the chief god of this city.
He also indicates that the Slavs originally did not give children divine names (as happened in ancient Greece), so the recognition of Radegast as a theonym would require the assumption of an exceptional situation.
[52] Radegast is also found[53] in the glosses falsified by Václav Hanka in the 19th century in the Czech-Latin dictionary Mater Verborum.
The legend first appears in 1710 in Sacra Moraviae historia sive Vita S. Cyrilli et Methodii by parish priest Jan Jiří Středovský.