Tertullian was a theologian and fierce defender of orthodoxy. He articulated an elegant and sophisticated doctrine of the nature of God and coined the term Trinity. His influence on Christian thinking is enormous, but also very significant was his gradual rejection of the Greek philosophical culture which the early church was formed within.
You can read a brief biography of his life here [though with a rather more positive interpretation than mine], and find his surviving works and more in the excellent Tertullian Project
I was struck in studying Tertullian (ironically) by the importance of heresy to sharpen and refine orthodoxy. If it had not been for the views of people like Marcion and Arius this elegant articulation of the Trinity may not have come into being. The church has tended to try to lock down orthodoxy through fear of error, but it is that very struggle for truth which spurs us to understand and better articulate what we know of God.