John was telling the people to get ready, judgment was coming
Q. In Matt 3.10 it says that "the axe is already laid at the root of the tree." What does that mean?
A. These words were spoken by John the Baptist as he was preaching along the Jordan, preparing people for the coming of the Messiah.
They were to get ready spiritually and repent from all the false teachings that they were hearing from the religious leaders.
The prevailing thought was that God wasn’t going to judge them because they were His children and the Temple was there and they were exempt from a few things, most notably faith in the Lord.
Now, not everyone believed this but John was speaking to a mixed crowd and some thought that because they were Jewish they had something special (3.9) and God would never do away with Israel.
Many knew the prophecies of Daniel and knew that Rome was the fourth kingdom predicted and believed that Messiah was coming.
As a result, he would do away with Rome and establish Israel as the head of the nations. What they didn’t see was that prophecy was going to take 2,000 years to be fulfilled and the Messiah was going to be cut off and the nation would be destroyed and scattered among the nations.
Rome had already conquered Israel and was already in the land administering Roman rule, although they allowed the Jews to practice their religion.
So, John was telling the people to get ready, judgment was coming and the "axe" (Rome) was already at the root of the "tree" (Israel).
In other words, the Lord has already moved in with His instrument of judgment (Rome/axe) and if the people did not turn to the Lord they were going to be cut down.
Matt 15.13 says that every plant that was not planted by the Lord was going to be "cut down" and that happened in 70 A.D. during the first Jewish revolt.