Preach true

"When he saw many Pharisees and Sadducees coming to his baptism, [John the Baptist] said to them : A race of vipers, who has warned you to flee from the anger that comes ? So produce fruit that is suitable for repentance." Matthew 3: 7-9

 
John the Baptist does not preach a theoretical message. He does not speak in vague terms to avoid scandalizing his audience, he preaches with conviction and severity.
The prophet speaks to everyone, without distinction of rank or class: to crowds, tax collectors for the Romans, soldiers, Pharisees, Sadducees, King Herod.
John the Baptist denounces sins and speaks of judgment in very strong terms, but at the same time he indicates the remedy. To prepare the people for the coming of the Messiah, he goes to the source of evils : the human heart, his pride and his evil thoughts, for "it is from the heart that bad thoughts, murders, adulteries, fornications, thefts, Testimonies, insults "(Matthew 15:19).
The heart of man can not be improved, despite all the attempts of reforms proposed by men. The prophet Jeremiah says : "The heart is deceitful above all things, and incurable: who knows it ?" (Jer 17:9). Man must be born again and begin an entirely new life. This new birth, of divine origin, is carried out by God, and the means is His Word and the Holy Spirit.
John the Baptist was only a man. He could not communicate the divine life, nor could he exercise judgment. It does not require its listeners to respect rules or privations. He simply tells them the first step to make : to repent by showing their sincerity by fruits.
The prophet baptized water as a sign of repentance, that is, of returning to God. He denounced vigorously what was false, but he indicated at the same time the solution and directed the eyes towards the Messiah : the one who would come after him would baptize of the Holy Spirit, the one who would introduce a life power into a new state of things.
The prophet speaks to the conscience of his hearers ; He takes them back, exhorts them, evangelizes them. In this, he is a model for every preacher.
 
Source (Please the Lord)