Matthew 3:10

And this is not all: And now also the ax is laid to the root of the trees. Therefore every tree which bringeth not forth good fruit is hewn down, and cast into the fire.

The ax has been placed, it is even now ready to begin its work of just retribution, of stern justice upon every spurious descendant of Abraham. Every tree which proves itself hopelessly barren cannot escape the near inevitable doom. And John makes use of careful phrasing. Not only is fruit demanded, which may, under circumstances, be unpalatable and even poisonous, but his condition is that the tree produce good fruit. Unless this demand is met, there is no other alternative: The useless tree is condemned to be firewood; the unbelieving Jew will be excluded from the kingdom of the Messiah.