He doesn't promise to save us from becoming sinners

Who are his people? We are eager to know who they are, and we are glad to find that his people need to be saved, and will be saved, for it is written, ‘He will save his people.’ It is not said, ‘He will reward his people for their righteousness,’ nor is it promised that he will ’save them from becoming sinners,’ but ‘He will save his people from their sins.’ . . .

If you are righteous in yourself, you are not one of his people. If you were never sick in soul, you are none of the folk that the Great Physician has come to heal. If you were never guilty of sin, you are none of those whom he has come to deliver from sin. Jesus comes on no needless errand and undertakes no unnecessary work. If you feel yourselves to need saving, then cast yourselves upon him, for such as you are he came to save.

C. H. Spurgeon, The Treasury of the New Testament, I:4-5, on Matthew 1:21.