Erasmus of Rotterdam is usually regarded as the foremost humanist scholar of his day, leading the quest to appropriate classical morality for Christianity from the front. His books centre around an attack of the over-emphasis placed on the cultic aspects of Christianity in the Church of his time, and a wish to have the focus of Christianity move back to morality. This was an attempt to renew Christianity rather than to demolish it.
Humanists were at this time involved in the translation and explication of Cicero and Seneca, the two classical authors taken to present the best moral standards for statesman and their advisors to live by. This so-called "mirror-for-princes" literature produced a whole number of books called The Prince before Machiavelli stole the show in 1532. Rulers were beseeched to be wise, clement, generous, and to be loved rather than feared, and they were to look to the glorious examples of past ancient princes as an example. This presented something of a problem for anyone with a habit of self-reflection.
The Erasmian paradox was this. Sixteenth century Europe had been around for fifteen hundred years since Christ, and had all the benefits of Divine revelation with the addition of the scholarship of Europe's greatest minds in the intervening time frame. However, they were still forced to look to examples from classical times to show them how to live good and moral lives. How could it be that the ancients had made great strides in moral philosophy consonant with Christianity, whereas Early Modern Europe - the inheritor of this and the greater legacy of Christ's Revelation - seemed to have regressed?
Thomas Aquinas had considered the relationship between religion and morality several centuries earlier, providing the answer of the scholastic tradition. He said that although reason would allow pagans to make great strides in moral philosophy, they would require Divine revelation to perfect it. Erasmus reformulated this question by pointing out that this was all very well, but that the opposite seemed to have happened in the case of Europe. Transcending a formulation of the question that concerned merely the nature of grace and its relationship to revelation, Erasmus' new form entered the more complex sphere of the relationship between morality and religion.
In the Enchiridion Militis Christiani (Manual of the Christian Gentleman), Erasmus gives an answer to his paradox in the course of explaining how one should live a life appropriate to the worship of Christ. He begins by splitting Christianity into two portions, the cultic/ritualistic (which can be called simply "prayer", but includes the sacraments) and "knowledge", which includes moral as well as intellectual knowledge. The two are complimentary and neither can get along without the other, and they both have different functions for the Christian.
"Prayer" essentially includes the ways of approaching God and communicating with him, and so is highly important. From the Bible, we know what are the proper ways to communicate with God and 'access' his Divinity through sacraments, prayers, and ritual. However, without the right knowledge of morality and how to live the good life such an approach to God was useless. We have to, Erasmus explains, form a compound of the qualities of Aaron (brother of Moses and the first High Priest of the Israelites, hence representing the cultic part of Christianity) and Moses (the law-giver, hence representing the moral, knowledgable part).
Erasmus was hence asserting an unbreakable link between religion and morality, and providing an answer to the problem of a Christian morality which transcended the scholastic one. Morality was something that came from within, through what Aristotle would call "right reason" - the appreciation of what was right and good in the world and the acting upon it. Virtuous pagans could quite easily attain this state, but without revelation they would be unable to use the tools of Aaron to approach God. Similarly, a people with knowledge of the cultic side would not get anywhere without an appreciation of the moral part of the equation. The problem in contemporary Europe was that there was too much stress on the ritualistic side of Christianity, which was not in itself a problem but became one when not combined with an appreciation and cultivation of the moral side. In this way Erasmus solved his own paradox.
Erasmus was a great friend of Thomas More, and dedicated his most famous work In Praise of Folly to him. It is hence not surprising that one possible reading of More's Utopia - indeed, a highly convincing one - sees this process at play. The virtuous pagans of Utopia live a life which seems in full accordance with Christian morality, and yet they have not received the revelation. But when they learn of Christ's teachings from Hythloday (the protagonist of the work, a European who has travelled to Utopia), they show interest in adopting the sacramental system and instituting a priesthood. Hence More's work was not a straightforward attack on the Church and an assertion of justification by virtue alone, but rather a plea for the reform of the Church and the remarriage of virtue to ritual. He shared with Erasmus this desire to reinvigorate the Christianity of his day.