In 1517 Martin Luther published his 95 Theses in defiance of the corruption of the Catholic Church. John Calvin first published Institutes of the Christian Religion in 1536. Reformist thinkers such as these encouraged a rejection of the Church's current laws. Among the most important were the denial of the authority of the Pope, a rejection of the unregulated capitalism within the structure of organized Christianity (the buying/selling of indulgences and clerical offices), and the questioning of the devotion to Mary, opting for a more personal relationship with God/Jesus. Renaissance Humanism influenced reformist church thinkers, which in turn influenced the development of Secular Humanism as we can see through the recapitulation of the Reformation by the Avant-garde artistic movements of the 19th and 20th Centuries.
First, with reference to the denial of the authority of the Pope, we find the Modernist/Avant-Gardist Oedipal relationship to predecessors. In both cases, we find the precursor (the Pope or any existing artistic movement) as something to be 'murdered'/removed for its prohibition of a deeper relationship with the origin. The Pope stands as a pre-existing husband to Christ, barring the Christian from an intimate relationship with Christ. [The Bridal Theology is reversed here in order to hold the Oedipal arrangement - perhaps it would be more accurate to utilize the Electra Complex.] In the same way, the Avant-garde artist sees pre-existing art as a barrier to his/her intimate experience with truth, authenticity and originality, thus as a rule prior movements must be rejected.
Second, with reference to the rejection of the buying and selling of indulgences and clerical offices, these acts of commerce were seen as belittling the meaning of receiving forgiveness and corrupting the purity of the officers of the church. For the Protestant, the hierarchy, if any, should correlate to the piety of the official. Similarly, throughout the history of Modernism, we see a continual denial of Bourgeoisie society/culture by various Avant-garde movements, each one rejecting capitalism as a barrier to reality in its own way, from the Dadaist to the Minimalist, Performance Artist to Street Artist. (Of course there is always the re-perversion of ideals, think Jim/Tammy Faye Bakker or Jeff Koons.)
Finally, and probably most importantly, as it feeds into the logic of the previous examples, the Protestant insists on a more personal relationship to Christ. The increase in literacy paired with a growing distrust of Church officials lead many to desire and demand that the individual be given a direct connection to God/Jesus through the scriptures and prayers. No longer did the patron feel a need to go to Mary for intercession, but instead could go straight to the source of salvation, Jesus. In art, this is most clearly seen in Post-modernism. Although earlier versions of the Modernist Avant-garde placed a premium on individuality, never had it been so personalized as with the rise of pluralism (of interpretation) -the infinite possibilities for individual-based meaning - and the mark of post-modernism, relative realities. With this new freedom, the cultural critic (artist) is freed from the pressures of adhering to a meta-narrative in the pursuit of his/her endeavors.