Complex sentence: Returning to the United States in 1845, he continued his missionary work by establishing St. John's Church in Baltimore, while at the same time studying medicine, and obtained his M.D. from the University of Maryland in 1847.
Split into simpler sentences: Returning to the United States in 1845, he continued his missionary work and established St. John's Church in Baltimore. At the same time, he studied medicine and obtained his M.D. from the University of Maryland in 1847.