The Lord now makes a fourth covenant.
Alma is told that he is to gather the Lord's "sheep." These "sheep" will be received into the church, and will be received by God. They will covenant through baptism, and the Lord's covenant is that through their covenantal baptism they will be forgiven of their sins.
Literary: The Lord uses the word "sheep" here. In a New World context this would be rather out of place. While one may discuss what other animal might be meant by "sheep" in some verses, the "sheep" here are clearly humans. This is a literary device, not a specific animal. It may or may not have been the precise word Alma understood, but it is the term used for similar meaning in the New Testament, and therefore is used here. It is understandable in its imagery, even though the imagery is more modern than ancient in this context.