After Christianity became a licit religion in the Roman Empire, the Church quickly became more secularized, that is, more worldly. According to Dietrich Bonhoeffer in his classic The Cost of Discipleship, what was lost was the costliness of grace. Bonhoeffer contends that the ancient Church did not lose its sense of of the costliness of grace altogether because it was preserved in monasticism. Indeed, those we revere as the Desert Fathers and (yes there were some) Mothers were driven to the desert largely by the phenomenon of the Church's rapid growth and equally rapid secularization. Bonhoeffer, taking his cue from historians of the ancient Church, marvels that the monastic movement did not result in a schism. Bonhoeffer wrote, "the Church was wise enough to tolerate this protest."
Over time, according to Bonhoeffer, even monasticism came to be relativized and was used by the Church "to justify the secularization of its own life." How monasticism came to be used this way was, instead of being a form of discipleship for everybody and, hence, the way of communal life for the Church, it came to be viewed "as an individual achievement which the mass of laity could not be expected to emulate." As a result of "limiting the application of the commandments of Jesus to a restricted group of specialists," Bonhoeffer insisted, "the Church evolved the fatal conception of the double standard - a maximum and a minimum standard of Christian obedience." So, when "the Church was accused of being too secularized, it could always point to monasticism as an opportunity of living a higher life within the fold, and thus justify the other possibility of a lower standard of life" for Christians who were not monastics.
"By and large," Bonhoeffer concluded, "the fatal error of monasticism lay not so much in its rigorism... as in the extent to which it departed from genuine Christianity by setting up itself as the individual achievement of the select few... "
Bonhoeffer, who was a Lutheran, saw in Martin Luther's movement from the world to the cloister and back to the world as precisely the movement required for a needed correction, wrote: "It is a fatal misunderstanding of Luther's action to suppose that his rediscovery of the gospel of pure grace offered a general dispensation from obedience to the command of Jesus, or that it was a great discovery of the Reformation that God's forgiving grace automatically conferred on the world both righteousness and holiness. On the contrary, for Luther the Christian's worldly calling is sanctified only in so far as that calling registers the final, radical, protest against the world" (emboldening and italicizing emphasis mine).
It is no wonder that so many spiritual writers today are expounding those ancient Christian practices preserved in monasticism and applying them so fruitfully to daily living for all Christians. Several works come immediately to mind: Debra Farrington's Living Faith Day By Day: How the Sacred Rules of Monastic Traditions Can Help You Live Spiritually in the Modern World, Esther de Waal's Seeking Life: The Baptismal Invitation of the Rule of St. Benedict, Scot McKnight's Fasting, which is part of Thomas Nelson Publisher's "The Ancient Practices" series, and Richard Foster's still relevant and valuable Celebration of Discipline. I could list many, many more
From the fifth chapter of Lumen Gentium (Dogmatic Constitution on the Church) is entitled "The Universal Call to Holiness":
Thus it is evident to everyone, that all the faithful of Christ of whatever rank or status, are called to the fullness of the Christian life and to the perfection of charity; by this holiness as such a more human manner of living is promoted in this earthly society. In order that the faithful may reach this perfection, they must use their strength accordingly as they have received it, as a gift from Christ. They must follow in His footsteps and conform themselves to His image seeking the will of the Father in all things. They must devote themselves with all their being to the glory of God and the service of their neighbor. In this way, the holiness of the People of God will grow into an abundant harvest of good, as is admirably shown by the life of so many saints in Church history (par. 40)