29 “Let them be blotted out from the book of the living”. For had they been some time written therein? Brethren, we must not so take it, as that God writes any one in the book of life, and blots him out. If a man said, “What I have written I have written,” concerning the title where it had been written, “King of the Jews:” does God write any one, and blot him out? He foreknows, He has predestined all before the foundation of the world that are to reign with His Son in life everlasting. These He has written down, these same the Book of Life does contain.
Lastly, in the Apocalypse, what says the Spirit of God, when the same Scripture was speaking of the oppressions that should be from Antichrist? “There shall give consent to him all they that have not been written in the book of life.” So then without doubt they will not consent that have been written. How then are these men blotted out from that book wherein they were never written? This has been said according to their own hope, because they thought of themselves that they were written.
What is, “let them be blotted out from the book of life”? Even to themselves let it be evident, that they were not there. By this method of speaking has been said in another Psalm, “There shall fall from Your side a thousand, and tens of thousands from on Your right hand:” that is, many men shall be offended, even out of that number who thought that they would sit with You, even out of that number who thought that they would stand at Your right hand, being severed from the left-hand goats: not that when any one has there stood, he shall afterwards fall, or when any one with Him has sat, he shall be cast away; but that many men were to fall into scandal, who already thought themselves to be there, that is, many that thought that they would sit with You, many that hoped that they would stand at the right hand, will themselves fall.
So then here also they that hoped as though by the merit of their own righteousness themselves to have been written in the book of God, they to whom is said, “Search the Scriptures, wherein ye think yourselves to have life eternal:” when their condemnation shall have been brought even to their own knowledge, shall be effaced from the book of the living, they shall know themselves not to be there. For the verse which follows, explains what has been said: “And with just men let them not be written.” I have said then “Let them be effaced,” according to their hope: but according to Your justice I say what?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)