Systematic theology is any study that answers the question, "What does the whole Bible teach us today?" about any given topic.
Wayne Grudem (Systematic Theology, pg. 13)
"Wherefore all theology, when separated from Christ, is not only vain and confused, but is also mad, deceitful, and spurious; for, though the philosophers sometimes utter excellent sayings, yet they have nothing but what is short-lived, and even mixed up with wicked and erroneous sentiments."
~ John Calvin (1509 – 1564)
We need theology for the sake of people. Theology is the application of the Word by persons to the world and all areas of human life.
John Frame (Salvation Belongs to the Lord, pg. 72)
Theology...the church's reflection on God's performative action in word and deed and its own participation in the drama of redemption.
Michael Horton (Covenant and Eschatology, pg. 4)
Theology is the intellectual, relational and spiritual act of knowing God through the disciplined and communal practice of understanding and applying His word to all areas of life.
Every Christian should be a theologian.