Answer:
The mystery surrounding Nikitin's journey was whether he changed faith to Islam
Explanation:
From the story attached, Nikitin, who was a christian Russian merchant went on a journey from Russia to Persia and then to India. This journey was recorded (in writing) by he himself in a book called Voyage Beyond Three Seas. One subject of controversy or uncertainty was whether he changed his faith or not during the course of his journey as he practiced/observed several Islamic pillars of faith (like fasting during the month of ramadan) which in itself doesn't make him a Muslim as argued by some scholars but could also meant he changed faith as argued by others.
He died on his way back home and perhaps made it difficult to ascertain his faith during the course of the journey.
