The following is an excerpt from The Soul Winner by Charles Spurgeon. It is one of the best books on evangelism that I have read.
I am sure you would say, first of all, that a man who is to be a soul-winner must have holiness of character. Ah! how few who attempt to preach think sufficiently of this! If they did , it would strike them at once that the Eternal would never use dirty tools, that the thrice-holy Jehovah would only select holy instruments for the accomplishment of His work. No wise man would pour his wine into foul bottles; no kind and good parent would allow his children to go to see an immoral play; and God will not go to work with instruments which would compromise His own character. Suppose it were well known that, if men were only clever, God would use them, whatever their character and conduct might be; suppose it were understood that you could get on as well in the work of God by chicanery and untruthfulness as by honesty and uprightness, what man in the world, with any feeling, would not be ashamed of such a state of affairs? But, brethren , it is not so. There are many in the present day who tell us that the theatre is a great school for morals. That must be a strange school where the teachers never learn their own lessons. In God’s school, the teachers must be masters of the art of holiness. If we teach one thing by our lips and another by our lives, those who listen to us will say, “Physician, heal thyself.” “Thou sayest, ‘Repent.’ Where is thine own repentance? Thou sayest ‘Serve God, and be obedient to His will.’ Do you serve Him? Are you obedient to His will?” An unholy ministry would be the derision of the world and a dishonour to God. “Be ye clean, that bear the vessels of the Lord.” He will speak through a fool if he be but a holy man. I do not, of course, mean that God chooses fools to be His ministers; but let a man once become really holy, even though he has but the slenderest possible ability, he will be a more fit instrument in God’s hand than the man of gigantic acquirements, who is not obedient to the divine will, nor clean and pure in the sight of the Lord God Almighty.