Exodus 32:33

And the LORD said unto Moses, Whosoever hath sinned against me, him will I blot out of my book.

And the Lord said unto Moses,.... In answer to his request:

Whosoever hath sinned against me, him will I blot out of my book; not that anyone that is really in the book of life is ever blotted out, or that anyone predestinated or ordained to eternal life ever perish: but some persons may think themselves, and they may seem to be written in that book, or to be among the number of God's elect, but are not, and turn out obstinate impenitent sinners, and live and die in impenitence and unbelief; when it will appear that their names were never written in it, which, is the same thing as to be blotted out of it, see Psalms 69:28. Now by this answer the Lord does not absolutely refuse the request of Moses with respect to the people, though he does with regard to himself, and the blotting his name out of his book; and it is plain, by what follows, he meant to show mercy to the people, since he bids Moses go and lead them on towards Canaan, and promises an angel to go before them; though he reserves to himself a liberty to chastise this people for this sin, as he should have opportunity, along with others.