Mosiah 13:25-35

27  And now ye have said that salvation cometh by the law of Moses.  I say unto you that it is expedient that ye should keep the law of Moses as yet; but I say unto you, that the time shall come when it shall no more be expedient to keep the law of Moses.
28  And moreover, I say unto you, that salvation doth not come by the law alone; and were it not for the atonement, which God himself shall make for the sins and iniquities of his people, that they must unavoidably perish, notwithstanding the law of Moses.
29  And now I say unto you that it was expedient that there should be a law given to the children of Israel, yea, even a very strict law; for they were a stiffnecked people, quick to do iniquity, and slow to remember the Lord their God;
30  Therefore there was a law given them, yea, a law of performances and of ordinances, a law which they were to observe strictly from day to day, to keep them in remembrance of God and their duty towards him.
31  But behold, I say unto you, that all these things were types of things to come.

Mosiah 13:27-31 (Emphasis mine)

Yes, the law of Moses leads to salvation, Abinadi tells them, but they day will come when this will not be so.  Salvation can only come through the atonement.  The atonement must be done by God himself.  

He then answers a question - "Why do we have the law of Moses?"  It was easy for them to fall into sinful and wicked ways.  In many ways, spiritually speaking, they were children.  They needed a constant reminder of how they were to live; hence, the law of Moses.  

This was a performance oriented law.  There were daily observances that needed to be done in order to fulfill the law.  These constant observances were a way for them to always remember what the Lord expected of them.

The law was also a "type" of things to come.  Let's turn to the scriptures to get an idea of what Abinadi meant.

3  ¶ Speak ye unto all the congregation of Israel, saying, In the tenth day of this month they shall take to them every man a lamb, according to the house of their fathers, a lamb for an house:
4  And if the household be too little for the lamb, let him and his neighbour next unto his house take it according to the number of the souls; every man according to his eating shall make your count for the lamb.
5  Your lamb shall be without blemish, a male of the first year: ye shall take it out from the sheep, or from the goats:

Exodus 12:3-5 (Emphasis mine)

29  ¶ The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.

John 1:29 (Emphasis mine)

The Lamb, of course, represents Christ.  He was called the Lamb of God, and he was without blemish (sin).

9  And Moses made a serpent of brass, and put it upon a pole, and it came to pass, that if a serpent had bitten any man, when he beheld the serpent of brass, he lived.

Numbers 21:9 (Emphasis mine)

14  ¶ And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up:
15  That whosoever believeth in him should not perish, but have eternal life.

John 3:14-15 (Emphasis mine)

17  ¶ Now the LORD had prepared a great fish to swallow up Jonah.  And Jonah was in the belly of the fish three days and three nights.

Jonah 1:17 (Emphasis mine)

Christ was in the tomb three days and three nights.

32  And now, did they understand the law?  I say unto you, Nay, they did not all understand the law; and this because of the hardness of their hearts; for they understood not that there could not any man be saved except it were through the redemption of God.
33  For behold, did not Moses prophesy unto them concerning the coming of the Messiah, and that God should redeem his people?  Yea, and even all the prophets who have prophesied ever since the world began—have they not spoken more or less concerning these things?
34  Have they not said that God himself should come down among the children of men, and take upon him the form of man, and go forth in mighty power upon the face of the earth?
35  Yea, and have they not said also that he should bring to pass the resurrection of the dead, and that he, himself, should be oppressed and afflicted?

Mosiah 13:32-35 (Emphasis mine)

Even though they were given a performance law, the people still did not understand the law and its purpose.  Moses prophesied of the coming of Christ; all prophets have basically taught about the coming of Christ.  Yet, they  people still didn't get it.  They hardened their hearts and ignored the words of the prophets. 

9  He sent redemption unto his people: he hath commanded his covenant for ever: holy and reverend is his name.
10  The fear of the LORD is the beginning of wisdom: a good understanding have all they that do his commandments: his praise endureth for ever.

Psalms 111:9-10 (Emphasis mine)

9  Receiving the end of your faith, even the salvation of your souls.
10  Of which salvation the prophets have enquired and searched diligently, who prophesied of the grace that should come unto you:

1 Peter 1:9-11(Emphasis mine)

So we learn that salvation comes through the law of Moses for a time, but that God himself would come to Earth and sacrifice His life for our sins.  It will ultimately be through the atonement that all will be saved.