Many people believe that the basis and source of defending the faith is different than that of knowing the faith. That is, they believe that we do systematic theology by appealing to the Scriptures, and we defend our faith by appealing to reason or philosophy (since unbelievers don’t accept the Bible as the Word of God).

The distinctive of the presuppositional (or transcendental) approach to apologetics developed most extensively by Cornelius Van Til is its demand that Scripture be the starting point for all human knowledge, and that it be maintained as such in a believer’s defense of the faith against attacks by unbelievers.

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…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.

It should not be forgotten in this connection that the minister’s duty is increasingly that of an apologist for Christianity. The general level of education is higher than it has ever been…If the minister would be able to help his young people, he must be a good apologete, and he cannot be a good apologete unless he is a good systematic theologian.

In conclusion, we should observe that just as a thorough knowledge of the system of truth in the Scripture is the best defense against heresy, so it is also the best help for the propagation of the truth…

And then Van Til starts stepping on toes:

The church will have to return to its erstwhile emphasis upon its teaching function if it is to fulfill its God-given task of bringing the gospel to all men. Its present recourse to jerky evangelism as almost the only method of propaganda is itself an admission of paupery…Revivalists ought to make themselves unnecessary as quickly as possible.

On this last point, Van Til is contrasting the modern evangelical and fundamental church’s tendency to reduce its purpose simply to training for evangelism, and jerky evangelism at that. I believe what he meant by “jerky evangelism” is an approach to evangelism where the message is divorced entirely from the rest of Christian doctrine and is reduced to a “get saved” message, followed by a “here’s how to get people saved” view of the Christian life. He was pleading for a return to a comprehensive instruction in sound doctrine that would provide the foundation for both a deep and strong faith, and a solid foundation for a robust defense of the faith against attacks. As a result, he calls for the swift elimination of revivalists who preach a denuded gospel that is insufficient for apologetics.