Carnell put forward a fixed approach, combining the classical approach, underlining the law of contradiction. He defends the proposition that logic must be valid because its denial is self-defeating. On the otherside, he rejected the idea that theism could be deductively proved. Therefore, he challenges Thomism's belief of a deductive approach, and finds that it cannot be validly proved.

Natural knowledge of God's existence doesn't take a person very far, not far enough. Thus, he centers on an evidentialist, experiential approach. Here he latches onto systematic consitency, the lack of internal contradiction coupled with external agreement in one's experience.

He proposed in other works and this that one's apologetic approach should mirror the times, as he stated: "an apologist must play it by ear."