He reminded them of all of the things that God had done for them, and how ungrateful they were.
Now it came to pass that Alma began to deliver the word of God unto the people, first in the land of Zarahemla, and from thence throughout all the land. 5.1
And behold, I say unto you, they were delivered out of the hands of the people of king Noah, by the mercy and power of God. 5.4
And behold, after that, they were brought into bondage by the hands of the Lamanites in the wilderness; yea, I say unto you, they were in captivity, and again the Lord did deliver them out of bondage by the power of his word; and we were brought into this land, and here we began to establish the church of God throughout this land also. 5.5
And now behold, I say unto you, my brethren, you that belong to this church, have you sufficiently retained in remembrance the captivity of your fathers? Yea, and have you sufficiently retained in remembrance his mercy and long-suffering towards them? And moreover, have ye sufficiently retained in remembrance that he has delivered their souls from hell? 5.6
(Much more of the same:) 5.7-21He really tries to make them feel guilty. My favorite line is:
And now I ask of you, my brethren, how will any of you feel, if ye shall stand before the bar of God, having your garments stained with blood and all manner of filthiness? Behold, what will these things testify against you? 5.22I would think that Jesus would be proud of their bloody clothes, since his own robes were dipped in blood. But you never know.
There's not much else of note in this speech. In verses 52 and 56, there is a nice description of Hell:
And again I say unto you, the Spirit saith: Behold, the ax is laid at the root of the tree; therefore every tree that bringeth not forth good fruit shall be hewn down and cast into the fire*, yea, a fire which cannot be consumed, even an unquenchable fire. Behold, and remember, the Holy One hath spoken it. 5.52
And finally, all ye that will persist in your wickedness, I say unto you that these are they who shall be hewn down and cast into the fire except they speedily repent. 5.56A lot of this chapter is "borrowed" from the gospels of Matthew and John--which is interesting, because this is taking place before they were written. God works in mysterious ways. But other than that, there isn't much going on this chapter. God is great, you are wicked, repent, repeat. If you want to read Alma's preaching in it's entirety, you can do so here. I already did, and I wouldn't recommend it :)
*And now also the ax is laid untothe root of the trees: therefore every tree which bringeth not forth good fruit is hewn down, and cast into the fire. Matthew 3:10