William Carey is an example of someone who make a career change to full-time ministry. This is not to say that all believers must be in full-time vocational ministry but as an example of someone who had training in other skills and deciding to go a different route to follow God. William Carey was training as a shoemaker in the middle of England. He was converted to Anglicanism and enthusiastically began studying Greek so he could study the New Testament in the original language. He became passionate about seeing all Christians working to fulfill the Great Commission and eventually organized a missionary society. More about Carey can be read at http://www.christianitytoday.com/ch/131christians/missionaries/carey.html?start=2.