Mphamvu Ya Mau A Mulungu

How is God's Word effectual in every way? How can it be said that God's Word does not return unto Him void, when there are many who do not believe Him, or the record that He has given of His Son? In what way do we see in the example of the conversion of the Ethiopian Eunuch, that it is God bringing His Word home to the heart of the sinner that converts that sinner, and NOT the word of the preacher?