In By Faith, Not By Sight: Paul and the Order of Salvation, Richard B. Gaffin Jr. writes:
Viewing Paul as a theologian, in the way we have, prompts a couple of observations on the much-mooted mater of the relationship between biblical and systematic theology. First, in exploring Paul’s theology, as an aspect of doing biblical theology, we should be aware that we are involved in doing systematic theology, or better, that our biblical-theological explorations will inevitably have systematic-theological repercussions. This is so because systematic theology ought to be radically non-speculative in the sense that its very existence depends upon sound biblical interpretation. Exegesis is its lifeblood, so that the method of systematic theology is fundamentally exegetical (15)

