In an earlier post, I defined medieval monasticism. Here I shall give more detail on the relations between the monks and the powerful leaders of society.
First of all, it is crucial to keep in mind that "the church" and "secular society" were not separate, discrete units. They constantly interacted. Indeed, church leaders, the bishops and abbots, were frequently the brothers and cousins of secular leaders. Those electing church leaders just took it for granted that those coming out of the leading families were more suited to command.
This did not however mean that church leaders were just the pawns of the powerful. Instead, for much of the Middle Ages, it worked the other way. A lord who misbehaved was likely to be subject to an extremely serious talk from an extremely close relative in the church.
Probably the majority of monks were also from the upper levels of society. Monasteries normally expected an entry gift, to pay for the upkeep (for life) of the new monk, and peasant families would not have felt they could spare any children. This did not mean of course that noble families were disposing of "extra" children, for giving a son or daughter to the church was always treated as a sacrifice, giving one's most precious possession to God.
These child offerings (called "oblates") were the most common sort of medieval monk or nun, brought up in the cloister, rarely if ever seeing relatives again. But some twelfth-century monasteries did not take child oblates, requiring instead adults who made the decision themselves.
Young knights frequently joined such houses, filled with religious enthusiasm, as excited to be giving up everything for God as they might, in other circumstances, have been excited to go on Crusade or to a tournament or to war. They would have to know Latin already to join, meaning a good education was required. Young adult peasants might experience the same enthusiasm, but without an education they could only become sort of halfway monks, conversi as they were called, who ended up doing a lot of the agricultural work.
Few men "converted" to the religious life in their middle years, though women might if widowed. (One spouse really could not enter the cloister while the other stayed in the world.) In old age, both men and women often "took the habit" as they felt themselves dying, as a last attempt to atone for their sins. Complications arose if they recovered from what they had thought was a fatal illness and decided they had changed their minds--for you could not change your mind about an oath to God.
As well as becoming monks and nuns themselves, nobles were the biggest donors to the monasteries, hoping to gain the favor of a monastery's saints through their generosity. They might even found a brand new monastery. The image above is of St.-Etienne of Caen, the monastery William the Conqueror, duke of Normandy, founded in the wake of his 1066 conquest of England, to try to make amends for the deaths of so many people.