John Wesley's understanding of ordination, and the reality and function of ministerial orders, came from a combination of sources, but in his journal he references two written sources in particular: Bishop Samuel Stillingfleet's, Irenicum, and Account of the Primitive Church, written by Lord Peter King (Wainwright, Methodists in Dialogue). Two important insights emerged for Wesley as a result of these two books, along with the study of the New Testament itself.
The first was that presbyters (elders) and bishops differed in their offices by function. They were not two separate offices in kind. Thus bishops were presbyters who functioned as superintendents over the church. Since bishops remain presbyters they were not to be ordained as bishops; they were instead consecrated for their task as overseers. Historically, the terms ordination and consecration have been used interchangeably, but they have also at times been differentiated. If the episcopacy was not an ordained office separate from deacons and elders, then the two terms speak of different things. Ordination focuses on the character of the office itself; consecration on the function of the office.
Second, Wesley became convinced that the New Testament did not dictate the structure by which the church was to order itself. The three-fold order of ministry (deacon, elder, bishop) developed gradually. Wesley writes, "...neither Christ or His apostles prescribed any particular form of church government, and that the plea for the divine right of Episcopacy was never heard of in the primitive Church" (Letter to James Clark, 1746).
However various church traditions have interpreted the New Testament and applied it to ordination, Dennis Campbell reminds us that "ordination in Methodism derives from his [Wesley's] judgments and actions" (The Yoke of Obedience). Wesley always believed that only the duly ordained clergy could administer the sacraments. The problem, by way of reminder, was how to make sure that the Methodists in America had enough clergy to administer the sacraments on a constant basis. On the basis of his study of the New Testament on ministerial orders, the influence of others who thought and and wrote about the matter, and on the practical need for ordained clergy in America, Wesley would make a momentous decision that would change the course of the Methodist movement and set it on a course away from a reform movement within Anglicanism to an autonomous denomination.
Methodism in America was fated to become the Methodist Church.