According to James Dunn, it is someone who:
Belongs to that group of Christians who have seen it as part of their calling to articulate their faith in writing and to instruct others in their common faith, and who have devoted a considerable portion of their lives to so doing (The Theology of Paul the Apostle, 2).
And for Dunn, Paul is “the greatest Christian theologian of all time” (3, italics original).
What do you think of this definition of a theologian? Would you want to modify it in any way? How does this definition relate to the work of the pastor? Is the pastor chiefly a theologian? Or something else?