The "Unction" Home City Card (added with The WarChiefs expansion), available for a level 40 Spanish Home City, gives Missionaries an aura that increases the attack of friendly units in a radius of 24 around them by 4.5%, and stacks with each other. Note that the extra attack is added to the unit's current attack, not its base attack, making it exceptionally powerful. The aura only affects units directly under the Spanish player's control; allied units (whether human or AI) and the Missionaries themselves are not affected, but the aura does affect ships as well.
Since the Unction aura does not increase hit points, the enemy may sometimes use hit-and-run tactics or units with large splash damage. Therefore, Missionaries' greatest defense are their Unction aura, their speed with Mission Fervor, and their armies' line of sight using the "TEAM Inquisition" card and the Town Watch improvement.
Spanish priests and monks established missions all over the New World. The selection of a site wasn't based simply on divine inspiriaton; it was a complicated process involving weeks, months, and even years of navigating bureaucracies and scouting out sites with sufficient water and good defensive qualities. It was very important that missions, forts, and towns had access to good water, because once established, they were largely on their own to look after their needs.
Missionaries, like the Italian Jesuit Eusebio Kino, oversaw the establishment of several missions. Father Kino, who explored and traveled with the Spanish, established dozens of missions including the missions at Nuestra Senora de los Dolores, San Xavier del Bac, Guevavi, and Tumacacori. He was known for a policy of kindness towards Native Americans, and often established peaceful relationships with groups of Native Americans near his missions.