MONASTICISM
Monasticism implies the existence of a discipline that can bring its practitioners closer, by their own efforts, to eternal life. The discipline is called 'asceticism'. Although nowadays we tend to think of asceticism uniquely in terms of renunciation it is worth remembering that the Greek word askesis actually means 'exercise.' It is a word that would be used for the exercises done by athletes and monastic literature (following an example set by St Paul - 1 Cor 9:24) often compares the work of the monks to the work of athletes.
The discipline claims its origin in the commandments of Christ, most obviously his reply to the question of the rich young man, '"What must I do to inherit eternal life?" ... "If you would be perfect, go, sell what you possess and give to the poor and you will have treasure in heaven; and come, follow Me.' (Mark 10:17 and Matthew 19:21). It entails a rejection of what would normally be considered the social and political virtues - consideration of one's own personal honour, loyalty to the family, loyalty to the nation. We in the West tend to associate the religious orders with good deeds, hospitals, schools etc, but in its essence the calling of the monk is, at least on the face of things, socially useless, concerned primarily with the individual's own salvation. The movement had its origins in the eremitic movement, the departure to the desert to lead a solitary life. This was not a matter of seeking a pleasant contemplative life in the tranquility of a monastery garden. The conditions of life in the desert were difficult. The hermits went to to the desert to do combat with the devil since that is where he was thought to be at his strongest. The battlefield was the hermit's own body. The manifesto of the battle was the book of Psalms which, to the profane eye, reads as a series of battle hymns. David, the warlike King, was to the monks, a spiritual master teaching the principles of the 'invisible combat'.
The hermits formed into monastic communities simply because of the difficulty, the near impossibility, with the real risk of madness, of the solitary life. But the ideal of the solitary life is embodied in the very word 'monk', 'monachos', meaning 'single', and the community life of the monks was seen as a necessary training for those few who wanted and were able to pursue what was considered to be the higher life of the solitary hermit (the same is true in Buddhism).
Although some of the lives of the saints suggest an earlier origin of the idea (and there is a hint of it in Hebrews 11:37-8), the movement took off in the fourth century at the very moment that Rome adopted Christianity, when Christianity for the first time offered good career opportunities as well as the possibility of building a great Christian culture. On the face of it this departure of many of the most earnest Christians would seem to have been, both socially and politically, a very undesirable development. The one Christian tendency that does not seem to have been tempted by monasticism was Arianism - denial of the divinity of Christ - and this may have been a reason why, through the fourth century, in defiance of the resolutions of the Council of Nicaea, Arianism was encouraged, in the end unsuccessfully, by the Emperors (and I have a notion that it may eventually have fed into the refusal of monasticism in Islam).
The monastery embodied a total commitment to Christ that was implicitly a reproach to the less than total commitment of the rest of us. It attracted the most serious members of society who might otherwise have been good soldiers or administrators. The monks and nuns devoting their lives to nearness to God could acquire a charismatic authority - perhaps analogous to the authority in other societies of shamans, also believed to be close to an extra-human reality - independent of the merely political authority of the government or of the church hierarchy (in the Roman church centred in Constantinople it became the rule - I'm not sure when - that Bishops could only be taken from the ranks of the monks). But despite their apparent refusal of the society it was soon considered to be in the interests of the wider society to support them. To understand this it is necessary to have some notion of what a Christian society was. It was believed to be in itself an organic unity, the 'Body of Christ' (1 Cor 12:12-31). As such, each part had a role to play in relation to the whole. The monks were the praying part. This does not mean that the monks were encouraged to think they were engaged in a search for anything other than their own salvation. A belief that their prayers were a service to the wider society and had a special resonance with God would induce spiritual pride or, to use a very useful Russian word, 'prelest' - spiritual illusion. Nonetheless a virtue in one part of the body had its effects on the whole body. To vary the metaphor, the intense research of the monks after union with God was the leaven that raises the lump.
This of course opened the way for the abuses complained of by the reformers. You could pay the monks to do your praying or your penance for you. But open to abuse as this may be it has a logic based on the spiritual interdependence of society. If the struggle to achieve union with God ('theosis' or 'deification' in the technical language of the monks) requires a renunciation of the virtues necessary to maintain a society (including, for example, the warlike virtues), nonetheless the maintenance of the society is necessary to the wellbeing and tranquility of the monks. As the laymen do the sinning necessary to maintain the wellbeing of the monastery so the monks do the penance necessary to maintain the wellbeing of the laymen.