The most important thing for them to know is the Bible, specifically the attributes of God.
I remember Dr. Sproul saying that the ultimate paradigm through which we see every other area of theology is the person, character, and attributes of God. It is the template through which we have a Christian worldview. Everything is seen through who God is. The attributes of God bring our Christology and pneumatology into focus as well. Theology proper is the most important area of theology as it emerges from Scripture—the existence, person, being, essence, attributes, triunity, and eternal decree of God.