Complex sentence: He was ordained in 1946, first serving as an assistant at St. John's, Lafayette Square in Washington, DC, and then as Rector of Christ Church in Poughkeepsie, New York, before becoming head of the Department of Religion and chaplain at Columbia University.
Split into simpler sentences: He was ordained in 1946, first serving as an assistant at St. John's, Lafayette Square in Washington, D.C., and then as Rector of Christ Church in Poughkeepsie, New York. He next became head of the Department of Religion and chaplain at Columbia University.