Casiodoro de Reina was one of the first Spanish Reformers to translate the Bible from the original Hebrew & Koine Greek manuscripts available to him at the time. A Catholic monk, Reina came under the influence of other Spanish Reformers who had published their own translations of the Bible thus, breaking with Catholic laws of the time:
"[it is illegal for anyone to translate the] Bible in Castilian romance [Spanish] or in any other vulgar tongue…and any other books of Holy Scripture in Castilian romance, French or Flemish or any other tongue which have prefaces, notes or glosses that reveal erroneous doctrines repugnant or contrary to our holy Catholic faith or to the sacraments of Holy Mother Church.” Index of the Spanish Inquisition, 1551.As per previous and contemporary Reformers, de Reina had to flee his home of Spain, seeking refuge in such places as England, Germany and eventually Geneva. It is here that until recently some scholars have written regarding his connection with free-thinkers such as Sebastian Castellion and his fellow Spaniard the anti-trinitarian Michael Servetus.
"Much of the opposition to Casiodoro came from the fact that he was friendly with people who were not acceptable to the champions of orthodoxy, particularly those of the Calvinist persuasion. It was also taken amiss that he had expressed disagreement with the execution of Servetus; whereas the successors of Calvin and his companions have since erected on the site of Servetus’ martyrdom an expiatory monument.
Detached from the strife of the period, we can readily understand that someone, who has escaped the risk of a like fate for similar reasons, would be horrified to find that what he thought to be a haven of gospel religion was capable of the same savagery as his native Spain.
During his stay in Geneva…there is evidence that he made an approach to Sebastian Castellion…it is possible for a man’s beliefs to undergo considerable modification during his lifetime…Hence, it is not inconceivable that in his early days in Geneva and London Reina had had leanings towards the doctrines of Servetus and others who questioned the traditional view of the Trinity. Corro’s letters indicate that Reina had at least considered them, and we know that he did not feel that such views should result in their holders being excluded from the fellowship of the church, but the Confession of Faith that he drew up for the Spanish Church of London gives no indication that he wished to question seriously the doctrine of the Trinity, even though the French Consistory wished Reina to amplify the statements on that doctrine.
Nevertheless, in admitting the need for the words ‘Trinity’ and ‘Person’, the Confession points out that the words are not taken from Scripture…
'...we confess the names of the Trinity and of Persons, of which the ancient Church Fathers did not see a great need to use in declaring what they felt against the errors and heresies of their time'…
Reina’s honesty in stating that neither the Trinity nor infant baptism is mentioned explicitly in Scripture seems to be a treacherous admission, which would have been better left unstated. It is indeed probably his honesty in speaking his mind, rather than any real deficiency in his beliefs, that brought disaster in the end. To admit that it was the writings of a condemned heretic that had been responsible for enlightening his mind about God, to state that since the Apostles nobody had spoken better than that same heretic, and to assert that it was a lack of Christian charity that had been responsible for his death at the state in Geneva, was merely to add fuel to the flames of suspicion that had already been kindled." Arthur Gordon Kinder, Casiodoro de Reina: Spanish reformer of the sixteenth century, 1975, pp. 82-84.