Theologians of the systematic kind (you know who) love to do deductions. The problem is that in their business of deduction, they have hidden axioms which may not be axioms at all.
Mathematicians on the other hand are not legally allowed to make deductions if the step does not conform to a deduction rule or is not supported by an axiom. So what are axioms? They are propositions that are self evidently true. For example in human logic, P v ~P (P or Not P) is an axiom (now to mess up your mind a bit, there is one logic system where this is not an axiom, think about the implication of that for a moment). This is evidently true, right? It is obvious a thing cannot be white and not white at the same time, etc.
Good Systematic Theologians stay with their axioms and proper exegesis. The axioms in systematic theology are gathered or sourced from Scripture. This is why I find it astounding when a systematic theologian does not do exegesis first, but rather wants me to go by rapid conceptualizations and make deductions from there. Statements in Scripture are axioms because God defines them to be God's Word and God's Word is true.
From what I have seen good Lutheran theologians stick close to their axioms(Scripture), and those who don't, ergo, are bad. For example, the BoC refuses to conclude there is double predestination though it affirms that God predestines believers. Logically the BoC may be thought of as strange in this regard. However, the BoC refuses to go the deductive route of asserting double predestination because the word "predestine" is only found in reference to believers and never in connection with un-believers. Hence, they are staying close to the statements of Scripture, their axioms. It just sound and responsible scholarship to be that way.