20 And the Prophet, seeing that the change of their solid substance into dust will deprive them of all share in the boon of fruit to be bestowed upon the happy man in season by the tree, has accordingly added: Therefore the ungodly shall not rise again in the Judgment. The fact that they shall not rise again does not convey sentence of annihilation upon these men, for indeed they will exist as dust; it is the resurrection to Judgment that is denied them. Non-existence will not enable them to miss the pain of punishment; for while that which will be non-existent would escape punishment, they, on the other hand, will exist to be punished, for they will be dust.
Now to become dust, whether by being dried to dust or ground to dust, involves not loss of the state of existence, but a change of state. But the fact that they will not rise again to Judgment makes it clear that they have lost, not the power to rise, but the privilege of rising to Judgment. Now what we are to understand by the privilege of rising again and being judged is declared by the Lord in the Gospels where He says: He that believes in Me is not judged: he that believes not has been judged already. And this is the judgment, that the light has come into the world, and men loved the darkness rather than the light.
Source: Homilies on the Psalms (New Advent)