He was a highly reputed theologian, mathematician and astronomer. He was well versed in canon law. Credited to him are 401 ecclesiastical canons - including the decrees of the councils of Nicaea, Constantinople, and Chalcedon.

Dionysius also translated many Greek works, and is considered the inventor of the Christian calendar which started a new chronology beginning with year of the Incarnation of Jesus Christ.

From the Catholic Encyclopedia:
Biography