Respectfully, I still think that God instigates conversion through the revelation of the Holy Ghost within them, and not the preachers sermon.
Romans 10:17
17So then faith cometh by hearing, and hearing by the word of God.
Matthew 4:4
4But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God.
Hebrews 4:12
12For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.