Some say evangelism is like tossing lit matches into upright kegs. Most kegs are filled with water, some are filled with gunpowder. C.S. Lewis was thinking gunpowder when he wrote this (Letters, 3:324–325):

My feeling about people in whose conversion I have been allowed to play a part is always mixed with awe and even fear; such as a boy might feel on first being allowed to fire a rifle. The disproportion between his puny finger on the trigger and the thunder and lightning which follow is alarming. And the seriousness with which the other party takes my words always raises the doubt whether I have taken them seriously enough myself.