Jamieson, Fausset and Brown on 2 Corinthians 12:4

4. unspeakable--not in themselves, otherwise Paul could not have heard them; but as the explanation states, "which it is not lawful ... to utter" [Alford]. They were designed for Paul's own consolation, and not for communication to others. Some heavenly words are communicable (Ex 34:6; Is 6:3). These were not so. Paul had not the power adequately to utter; nor if he had, would he have been permitted; nor would earthly men comprehend them (Joh 3:12; 1Co 2:9). A man may hear and know more than he can speak.