Mosiah 13:26 - 13:30

Mosiah 13:26 I say unto you, Nay; for if ye had, the Lord would not have caused me to come forth and to prophesy evil concerning this people.

I guess it was a rhetorical question as he answers it himself by saying they have not. If they had he would not be there accusing them of not doing so at this moment.

Mosiah 13:27 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.

On Abinadi goes on to explain the purpose of the law of Moses. Now they may think that salvation comes by the law of Moses and they are necessary to live by for the moment, but eventually they will be done away with.

Mosiah 13:28 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.

Because salvation does not come by the law of Moses alone. Salvation requires the atonement of a God to cover the sins of the people. If God does not do so, everyone will not be saved regardless if we live by the law.

Mosiah 13:29 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;

Because the children of Israel were of a disposition that they would only follow their own whims and not direction from God, they were given detailed laws to follow.
[Loving God and their neighbor was not easily followed by these people. They needed to be told exactly what not to do. Instead of living a principle like loving their neighbor and being nice they would wanted detailed regulations on how to behave at all times of the day and night.]

Mosiah 13:30 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.

All the laws given to them were designed to remind them through rituals of their God. As they preformed the requirements of the law of Moses hopefully their thoughts would be turned to God and what He had done to saved them from Egypt and sin.