Complex sentence: During the reign of Louis XIV, the missionaries multiplied and extended the field of their activities: the Sun King' gave them at once a material and a moral support, which the prestige of his victories and conquests rendered irresistible at the Porte.
Split into simpler sentences: During the reign of Louis XIV, the missionaries multiplied and extended the field of their activities. This monarch gave them at once a material and a moral support, which the prestige of his victories and conquests rendered irresistible at the Porte.