From Van Til:
It is but natural to expect that, if the church is strong because its ministry understands and preaches the whole counsel of God, then the church will be able to protect itself best against false teaching o every sort. Non-indoctrinated Christians will easily fall prey to the peddlers of Russellism, spiritualism, and all of the other fifty-seven varieties of heresies with which our country abounds. One-text Christians simply have no weapons of defense against these people. They may be able to quote many Scripture texts which speak, for instance, of eternal punishment, but the Russellite will be able to quote texts which, by the sound of them and taken individually, seem to teach annihilation. The net result is, at best a loss of spiritual power because of loss of conviction. Many times, such one-text Christians themselves fall prey to the seducer’s voice.
We have already indicated that the best apologetic defense will invariably be made by him who knows the system of truth of Scripture best. The fight between Christianity and non-Christianity is, in modern times, no piece-meal affair. It is the life-and-death struggle between two mutually opposed life-and-world views. The non-Christian attack often comes to us on matters of historical, or other, detail. It comes to us in the form of objections to certain teachings of Scripture, say, with respect to creation, etc. It may seem to be simply a matter of asking what the facts have been. Back of this detailed attack, however, is the constant assumption of the non-Christian metaphysics of the correlativity of God and man. He who has not been trained in systematic theology will often be at a loss as to how to meet these attacks. He may be quite proficient in warding off the attack as far as details are concerned, but he will forever have to be afraid of new attacks as long as he has never removed the foundation from the enemy’s position.
Van Til, Cornelius, An Introduction to Systematic Theology, Second Edition, 1974, R&R Publishing.
This is the foundation. The basis for an authoritative Christian practice is found in its foundation. Likewise the attacks we face today, especially in the fields of ethics and science, are challenges to the foundation of this world view. The first challenge is whether God even exists. After that is whether there is any evidence of having been created. Finally we are faced with a challenge to any sense of morality and ethics that might be brought to the public square.
But once we establish the legitimacy of the Christian framework we then gain the foundation for reaching into the fields of ethics, morality, and to be heard. If our faith is legitimate then it will stand up to criticism. But it must face those challenges head-on, else it might justifiably be dismissed as irrelevant.