Sentence:  He had gone into the Japanese navy , had been trained as an officer , had participated in one or two battles - he never went into detail regarding his military experience - and at the age of twenty-five , quite as a bolt out of the blue , he had walked into the mission as if he belonged here and had <WSD>become</WSD> a Christian . 
Meaning of the word marked with <WSD> tags:  undergo a change or development