John 4:41 And many more believed because of his own word;
Many had believed Jesus because of what the Samaritan woman had said to them. Now we see people believing because they have come and heard the Lord speak. He tells them the truth. Their hearts know that what He says is truth. They believe because of the Word.
You and I have His Word to share with people. The Word of God is the source of faith. As they hear it preached it will work in them. We must preach and teach the Word. We know that the Word will cause people to believe.