What does Preachers: Not To Preach Themselves mean in the Bible?

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Charles Spurgeon's Illustration Collection - Preachers: Not to Preach Themselves
We ascended the Sacro Monte at Orta, expecting to find that its holy hill was like that at Varallo, consecrated to representations of the life of Christ. To our disappointment we found that everything was to the honour and glory of St. Francis of Assisi, who nevertheless was represented as saying, 'God forbid that I should glory, save in the cross of our Lord Jesus Christ.' Too often when the preacher should think only of his Master, and labour only to set forth the Redeemer's glories, he is occupied with his own style and oratory, and so honours himself at the expense of his Lord.