Complex sentence: Founded in the fourth century by Gregory the Illuminator, it was one of the oldest monasteries in Armenia and one of the three most important sites for Armenian Christian pilgrimage and was one of the richest and most ancient institutions in Ottoman Armenia.
Split into simpler sentences: Founded in the fourth century by Gregory the Illuminator, it was one of the oldest monasteries in Armenia. It was also one of the three most important sites for Armenian Christian pilgrimage, and among the richest, most ancient institutions in Ottoman Armenia.