Saint Ambrose (339
. He spent his early career as the governor
, but was unexpectedly proclaimed bishop of Milan in 374
As bishop, St. Ambrose was immensely popular, and was well loved by his congregation. St. Ambrose was well known for his sermons, which were acclaimed to be "masterpiecs of Latin eloquence." It was in one of these sermons that Ambrose gained his most famous convert, St. Augustine.
Throughout his career, St. Ambrose fought a constant battle against pagans and heretics, particularly the Arians, using his influence on the emperor and the Roman senate to quell them.
A more thorough biography (from where much of the above comes from) is at britannica.com:
The Catholic Encyclopedia has an older, more complete biography at: http://www.newadvent.org/cathen/01383c.htm