The heart of the hearer of the gospel must move, either towards Christ or away from Him. Pastor Chan Kilgore puts it this way: "True gospel preaching changes the heart. It either awakens it or hardens it." (pg. 63)
This is what the gospel does. This is the why the gospel is Jesus is dangerous. When we hear the gospel word, we are opened up the the Word of God. We're subjected to God's Word reading us. We sit underneath it, and for the moment of our hearing, it rules us. It does not save all, but all who hear it are put in their place. This is dangerous, because the proclamation of God's Word goes only one way or another in the soul of a man, and one of those ways is the hardening of the man toward the grace of God. (pg. 70)
The proclamation of God's Word can't do nothing. It always has an effect. It never returns empty. Therefore, preach and teach it plainly, boldly and faithfully. Leave the results up to God.