In response to your question, you are right to say that logic is subservient to God. However, the unbeliever’s epistemological system, where his/her logic comes from, is not the same as the believer’s epistemological system. In fact, as Van Til would say, these two systems are wholly antithetical epistemologically. Though both the unbeliever and believer can both know that 2+2=4 in a logical basis, this does not mean that both parties are working on the same epistemological system of logic. It is by common grace that the unbeliever can use logic to be able to do arithmetic (or any scientific endeavor for that matter). For the believer, 2+2=4 is not based on some abstract principle of logic but we have a personal concrete universal as Van Til would say, showing that every fact in this world is a personal fact interpreted by a tri-personal God. However, though the unbeliever and believer may have antithetical epistemological systems, they have their “point of contact” is in the fact that they are both creation, which is the Christian metaphysic. Both unbeliever and believer live in God’s world, living by God’s rules as God gives them, whether one “admits” that God exists or not. The entire universe is narrated by the omniscient, transcendent God, which is the true metaphysic. It is because that the unbeliever and believer share the God-ordained metaphysic of creation, bearing the image of God, that they can have any point of contact in an apologetic discussion, even if these two parties are epistemelogically antithetical to one another. This is what Dr. Cassidy is referring to here by saying that we must “begin with the triune God,” unlike Classical Apologists who assume that the believer and unbeliever share not only a metaphysical common ground (which they do) but also an epistemological one (which they do not).