"Blessed art thou, Alma,; therefore, lift up thy head and rejoice, for thou hast great cause to rejoice; for thou hast been faithful in keeping the commandments of God from the time which thou receivedst thy first message from him. Behold, I am he that delivered it unto you
And behold, I am sent to command thee that thou return to the city of Ammonihah, and preach again unto the people of the city; yea, preach unto them. Yea, say unto them, except they repent the Lord God will destroy them."
To me, this scripture is a wonderful example of the Lord's love for the individual. Alma is on his way out of Ammonihah; he has been reviled, spat upon, and cast out of the city. The people refused to listen to his words, and even mocked him. Already he's having a pretty lousy day, but then the Lord sends an angel commanding him to return to Ammonihah and do some more preaching. It may have seemed to Alma that the Lord was saying: Yeah, yeah, I know you had a tough time; they spat on you and threw you out, but why not go back for some more?
But the Lord knew what kind of man Alma was; He knew what Alma needed to give him the strength to do what He asked. First, He sent the same angel that He sent when Alma and the sons of Mosiah were causing havoc among the members of the church, and had this angel tell Alma to lift up his head. Yes, the Lord knew Alma had been through some discouraging events in Ammonihah, but He reminds Alma that despite those events, he has reason to rejoice. Then, the Lord has that angel, who was a witness to Alma's actions (remember he told him to change his ways or be destroyed), say to Alma that he is blessed, that he has been faithful since the time he first visited him. This had to be very encouraging to Alma; imagine having an angel of God tell you that you've been faithful!
Why does the angel include the part about "from the time that thou receivedst thy first message?" I believe that rather than discourage Alma by reminding him of his past, the Lord included this in His message because he wanted to remind Alma that He knew Alma's past, yet still felt that Alma was capable of accomplishing the mission He had for him.
Did the Lord know Alma and his needs? Evidently! As soon as the angel delivered his message, Alma "returned speedily to the land of Ammonihah." (verse 18).