For a chain reaction to occur there has to be a spark, starting point, or what we are calling the "Lead Domino." This piece is pinnacle to the rest of the chain because without it the other parts wouldn't do what they were placed there to do. You could probably name some lead dominoes that have led you to your job, significant other, school, etc. Jesus is calling all Christians to be the lead domino to others by discipling and sharing The Gospel with them.