“The unenlightened understanding is unable to apprehend, and the unregenerate will is unable to believe. Vital force is lacking in these two principal faculties. What is needed at this point is life and force itself. Consequently, the author of spiritual life himself must operate directly, without the use of means or instruments, and outright give spiritual life and power from the dead, that is, ex nihilo. The new life is not implanted because man perceives the truth, but he perceives the truth because the new life is implanted. A man is not regenerated because he has first believed in Christ, but he believes in Christ because he has been regenerated. He is not regenerated because he first repents, but he repents because he has been regenerated.” (W.G.T. Shedd)
For more on the subject, visit the Systematic Theology page.