John said to the crowds that came out to be baptized by him, "You brood of vipers! Who warned you to flee from the wrath to come? Bear fruits worthy of repentance. Do not begin to say to yourselves, 'We have Abraham as our ancestor'; for I tell you, God is able from these stones to raise up children to Abraham. Even now the ax is lying at the root of the trees; every tree therefore that does not bear good fruit is cut down and thrown into the fire."
And the crowds asked him, "What then should we do?" In reply he said to them, "Whoever has two coats must share with anyone who has none; and whoever has food must do likewise." Even tax collectors came to be baptized, and they asked him, "Teacher, what should we do?" He said to them, "Collect no more than the amount prescribed for you." Soldiers also asked him, "And we, what should we do?" He said to them, "Do not extort money from anyone by threats or false accusation, and be satisfied with your wages."
As the people were filled with expectation, and all were questioning in their hearts concerning John, whether he might be the Messiah, John answered all of them by saying, "I baptize you with water; but one who is more powerful than I is coming; I am not worthy to untie the thong of his sandals. He will baptize you with the Holy Spirit and fire. His winnowing fork is in his hand, to clear his threshing floor and to gather the wheat into his granary; but the chaff he will burn with unquenchable fire."
So, with many other exhortations, he proclaimed the good news to the people.
In Biblical times, tax collectors were local agents of the Roman Empire. They would collect taxes from the local population for the Empire. As Wikipedia reports that the Romans essentially charged the tax collectors an upfront fee in return for the privilege of collecting taxes, and they in turn were allowed to collect money or goods from the populace. Tax collectors in the Bible were not well-liked by their fellow Jews, and it seems that they were often considered to have defrauded their fellows. Certainly, they were agents of empire. John ordered them to stop defrauding fellow Jews.
Similarly, the Roman soldiers that John the Baptist was preaching to were agents of empire. They were the primary occupiers. They likely committed a multitude of abuses against the Jewish population and other populations they occupied. Here, John orders them not to extort money from the Jews. Jane Schaberg, a feminist theologian, contends that Mary could well have been raped by a Roman soldier; whether you buy that or not, soldiers have committed rape throughout history and it's all but given that the Romans did too.
John the Baptist was Jewish and was mainly preaching to the Jews. I wonder what he would have said if he'd been an American. It's not too much of a stretch to think that he would demand an end to all unjust occupation. He would have said, you must close Guantanamo. You must not torture any enemy combatants, or send them to places where they will be tortured. When soldiers or military contractors are based abroad, they must not commit crimes against the local population, and if they do, they must be punished to the full extent of the law. When fighting the enemy, you must not harm innocent civilians.