For some time popular systematic theology has been dominated by an approach that presents our knowledge of God in terms of facts and propositions. In these propositional systems, statements are made about God which can be evaluated for truth value. Knowing the right propositions and being able to demonstrate their truth is sometimes considered to be the core of theological knowledge. "Systematic theology" is often taken to mean "abstracted theology" or "propositional theology".
The most basic problem with this approach is that God is a person, not merely a concept. God's divine personhood goes so far beyond our human personhood that we have trouble understanding it; but still God is not simply a mindless force but a person with mind and will, with love and desire. Given that God is not a mere concept, knowing God must extend beyond knowing the truth value of propositions. Limiting ourselves to propositions -- even propositions about God's personhood -- is an affront to the reality of that personhood. Would any of us imagine that someone could make a set of propositions about us as a person which could substitute for meeting and knowing us? But we have often made this mistake with God, and then theology becomes a means by which we keep God at a safe distance rather than coming to know him. God will not be a mere object of study. Given that God is a person, knowing God must include interacting with him as a person and knowing him as a person. God did not present himself to us as a set of propositions to be studied. He presented himself to us in Christ, the divine word coming to us not as a proposition but as a human person, Jesus Christ. These are the terms on which God would have us know him; beginning our systematic theology anywhere else but Christ is an affront to God's self-revelation.