The great purpose of the law of Moses was to keep its adherents clean and pure, that they might be susceptible to the witness of the Spirit and thus be prepared to accept the Messiah.
From what Abinadi has said, it appears that some in the Old World discerned the high spiritual meaning of the law as a witness of the Christ.
Many of the nation of Israel, though observant of the outward ordinances of the law, lost their understanding of its function and purpose through wickedness and thus hardened their hearts against the Christ when he came to them.