Harunobu was baptized by Alessandro Valignano in 1579. He took the baptismal name Protasius, and later took the name John when he received Confirmation. As a result of his conversion to Christianity, Harunobu started to receive weapons from the Portuguese, which strengthened the Arima clan. He also founded a seminary and training center for novices in his domain where, apart from the ordinary curriculum, students were also taught European music, painting and sculpture and the manufacture of organs and pocketwatch. In 1582 Harunobu teamed up with the Kyūshū Christian "daimiyōs" Ōtomo Sōrin and Ōmura Sumitada to send a Japanese embassy to the Pope in Rome, led by Valignano and represented by Mancio Itō.