John uses the image of cutting down trees and casting them into the fire to depict God’s judgment of the godless (Isaiah 10:33–34, Isaiah 66:24). The present tense wording implies that the judgment is coming very soon. Those who bear no good fruit will be counted as godless (see Matthew 3:8, Matthew 12:33). The image of the axe at the root of the tree implies the urgent need for repentance, and may also relate to the Old Testament image of the vineyard, where not being in the vine, but bearing good fruit saves you.
The cutting down of trees and throwing them into the fire speaks of the great judgment of God over all the godless (Isaiah 10:33–34, Isaiah 66:24). By casting the metaphor in the present tense, special emphasis is placed on the fact that the judgment is imminent. Each one who does not bear good fruit is thereby counted as godless (see Matthew 3:8; Matthew 12:33).1
The image used in Matthew 3:10 signifies the gravity of this particular moment. The general truth (that God can raise up children of Abraham from stones) now seems to be practically imminent. The message conveyed is that if you are descended from Abraham, but fail to live out of an attitude of repentance, you will be exterminated. This message is aptly conveyed by the image of an axe being laid at the root of a tree. The image of the tree is possibly derived from the Old Testament image of the vineyard. Not the vineyard, but the good fruit saves you from the fire.2
9 And do not presume to say to yourselves, ‘We have Abraham as our father,’ for I tell you, God is able from these stones to raise up children for Abraham.