10 Therefore, “Today if you will hear His voice, harden not your hearts”. O my people, the people of God! God addresses His people: not only the people of His which He shall not cast off, but also all His people. For He speaks in the corner stone to each wall: that is, prophecy speaks in Christ, both to the people of the Jews, and the people of the Gentiles. For some time ye heard His voice through Moses, and hardened your hearts. He then, when you hardened your hearts, spoke through a herald; He now speaks by Himself, let your hearts soften. He who used to send heralds before Him, has now deigned to come Himself; He here speaks by His own mouth, He who used to speak by the mouths of the Prophets.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)