Doing research for a writing, what would bet the highest-ranking - social, governmental, political - that a commoner could rise to in 1450s England with all the class differences in play?
Archbishop of Canterbury.
The church was long noted as the means by which those from less than exalted backgrounds could best make both a good living, and, potentially, their mark, both theologically and politically. A high proportion of successful prelates were the younger sons of prominent families – men who were unlikely to inherit significant amounts of land. Careers in the church offered such men livelihoods, and family connections the leverage to achieve higher office within the ecclesiastical community. These sorts of career decisions also applied to family members who faced more significant barriers to advancement than mere order of birth; thus at least two of the Canterbury archbishops of the medieval period – Ralph Nevill and John Stafford – were the illegitimate sons of noted families.
A second, and much smaller, group of eminent clerics made their way up the ecclesiastical ladder fro lower positions than that as a result of talent or patronage. Cathedral cities and monasteries typically provided opportunities for schooling for children from much more modest social backgrounds, and there was always demand from the royal government for educated men who could read and write. While those able to take advantage of such openings were rarely if ever from the very bottom rungs of society, sons of artisans and merchants could and did get themselves "talent-spotted" by superiors who valued their scholastic or administrative skills.
Very occasionally, a child from such a background might make it all the way. The best-known example from the medieval period was that of Thomas Becket, whose disagreements with Henry II, and eventual death within the precincts of Canterbury cathedral at the hands of a group of the king's knights, made him perhaps of the most famous of all English archbishops; he was the son of a successful London merchant. And Edmund of Abingdon, who became archbishop a few decades after Becket, was probably the son of a wool merchant.
Edmund's career offers a good example of the means of ascent that could be employed by men from more humble backgrounds in this period. His family background was sufficiently affluent to allow him to attend the University of Paris, after which he moved to the university at Oxford and became well known as an expert on Aristotle and a teacher of grammar. All university teachers this period were also ordained priests, and further alternating periods spent as a scholar at Paris and Oxford were interrupted by time spent in possession of various church benefices and preaching the crusade. Eventually one of Edmund's old university pupils, Walter de Gray, became Archbishop of York, and was able to use his influence to advance the career of his old master. Edmund ultimately became compromise candidate for the archiepiscopal throne at Canterbury in 1233.
Such a career was rare, but certainly far from unknown in the medieval period. While we often have only the barest details about the parentage and early careers of many eminent churchmen (and are not completely sure, for example, that Edmund's father really was a minor merchant), the archbishop who rose from the must humble background of all in the medieval period was almost certainly Walter Reynolds, who held the see at Canterbury from 1314-27. He is generally accepted to have been the son of a baker in Windsor, Berkshire, which meant that he began his church career in a town with very strong links to the English monarchy. This seems to have been critical; Walter became a clerk in the court of Edward I and there he met the king's son, the future Edward II. He quickly became a close friend of both the future monarch (who in 1309 described him as one who, "active in our service from our earliest youth, has came to enjoy our confidence ahead of others") and of Edward's lover, Piers Gaveston. As a result, he moved to take an administrative position as keeper of the young prince's wardrobe.
Reynolds's successful career, then, was entirely the result of royal favour and patronage. He was provided with the livings of a series of parishes (which he probably rarely if ever visited), which provided him with a good income, and in 1308 became Bishop of Winchester. He was named Chancellor of England in 1310, and Archbishop of Canterbury in 1314. He was sufficiently politically astute to switch sides with remarkable adroitness after Edward was overthrown by his wife and her lover, Sir Roger Mortimer, in 1327, preaching the text 'Vox populi vox Dei' (in which he justified the revolution and seems to have approved renunciation of homage to Edward II) only one day after the deposition of his old friend and patron.
Reynolds has an equivocal reputation. For Robert of Reading, he was
a man decidedly unclerkly, and so ill-educated that he was entirely unable to set out the form of his own name … Having ceremoniously received the insignia of an archbishop, he used them as an ox does its horns, in robbing churches and oppressing the religious, indulging in immoderate filthiness of lust.
For John of Trockelowe, on the other hand, the archbishop was a far more benign influence, the "man by whom those tribulations [of the church in England in this period] could best be assuaged." Modern historians have viewed him increasingly favourably. In the overall judgement of his biographer, J. Robert Wright,
A product of his own turbulent era, Reynolds tried to work with the crown rather than in direct opposition to it, prizing the virtues of moderation, harmony, and stability higher than a reliance on uncompromising standards in which he did not believe. Reynolds desired to see the king and realm at peace, and he used his influence to that end, even when it necessitated a politics based more on expediency than on ultimate principles. What has appeared as indecision to many commentators may in fact have been scrupulous and conscientious deliberation, probably influenced by the king's changing moods as well as by ... consistently cautious advice... Reynolds lived in a world of complex personal interests rather than in one of clear-cut constitutional conflicts. In spite of his evident personal limitations, his Canterbury appointment was consequently a political triumph for both Edward II and Clement V.
Sources
Oxford Dictionary of National Biography
J. H. Denton, ‘Canterbury archiepiscopal appointments: the case of Walter Reynolds’, Journal of Medieval History 1 (1975)