15 February 1537,
Eight years before Bernat Etxepare's book, Bishop Zumarraga wrote the oldest known text written in Basque. Born the Durangus, he was a Mexican inquisitor at the time of signing this text and in 1547 he was named first Archbishop of Mexico. There was no Christian religion that moved too far from the fire to burn.
It was discovered by Enrique Otte in 1979 and published. Only the end of the letter is in Basque (not complete). What has come to date is not the original of 1537, but a later copy.
Source:
Joxemiel Biker, eccentric classic, arront classic. (Pamiela). / Euskaltzaindia.