Robert Murray McCheyne, source unknown
Study universal holiness of life. Your whole usefulness depends on this, for your sermons last but an hour or two, your life preaches all the week. If Satan can only make a covetous minister a lover of praise, of pleasure, of good eating, he has ruined your ministry. Give yourselves to prayer, and get your texts, your thoughts, your words from God. Luther spent his best three hours in prayer.