The greatest natural intellect cannot receive this, he is ‘a natural man’. And you need a spiritual faculty to receive the wonderful truth about the two natures in the one Person; the outstanding doctrine about the Trinity; the whole doctrine of the incarnation and the atonement, and so on. This is spiritual truth and to the natural person it is utter folly, it is foolishness, as Paul says. So when the Holy Spirit does enable us to believe it, it must be something beyond the heightening of our natural faculties. It is not simply that He brings the truth of His great moral suasion to us. No, no. We need some new faculty, some new principle, and that is the very work that He does. He implants within us this new spiritual principle, this principle of spiritual vitality and activity, and it is as the result of this that the general call of the gospel comes to us in an effectual manner.
From the chapter on Effectual Calling in God the Holy Spirit