"If two men, one saved and one damned, can enjoy the same access to the same means, then the thing which distinguishes them is the faith of the one and the unbelief of the other. But that faith is itself a gift, and it is given in accordance with His sovereign will. The fact that the gift of faith is not inexorably given with every instance of a sacramental gift is what sets up the problem for us, and which is what makes historical evangelicalism a necessity." - Wilson, AC, p194.
In essence, this "problem" is the reason for Wilson's book.