Kevin J. Worthen, President of Brigham Young University, recently gave a talk in which he pointed out that Latter-day Saint scripture frequently uses the admonishment "Be of good cheer." The point of the gospel is to bring us good cheer; it is a contradiction to believe in repentance and to find it an unhappy prospect.
In the King James translation of the Bible, the phrase was used to translate the Greek imperative verb tharseo meaning "Take courage." "Courage" is one of the older meanings of cheer. The word itself descends from a Middle English word that means "face." It probably came to mean courage as well as to have its contemporary meaning, good feeling, because to have courage requires facing one's foes or problems with a lifted up head and a face that no longer droops.
Jesus uses the term "good cheer" in Matthew 9:2 when he tells a palsied man "Be of good cheer; thy sins be forgiven thee." In the face of the fear brought on by sin, the man can take courage in the forgiveness of sins that Jesus brings.
In his appearance to the disciples in the stormy waters of Galilee (Mt. 14:27; Mk. 6:50), Jesus' message was "Don't be afraid. It is I, not a ghost." He is a real, embodied person who has the power to save them. In John 16:33 the message is the same but broader: there will be tribulation in this world, but have courage in the face of that tribulation. Jesus' teaching brings peace because he has overcome not just the stormy seas, but the world.
In Acts 23:11, the Lord speaks to Paul in prison in Jerusalem admonishing him to take courage since he is going to have to bear testimony in Rome just as he has in Jerusalem. At first glance this is different from the other uses of the phrase because more than being encouraged, Paul is being warned: continue to have courage since you'll need it again in the future. You are going to find yourself in the same circumstances, persecuted and imprisoned for your testimony. Implicitly, though, the courage that the Lord admonishes is possible because Paul trusts in the gospel message of redemption.
The theme of the need for courage and the courage or cheer that the gospel brings continues in scriptures revealed through Joseph Smith. Nephi, the grandson of Helaman, prayed for the people who believed in Christ's coming to be spared from execution for their belief. The Lord answered his prayer telling him "Be of good cheer; for behold the time [of the Lord's coming] is at hand" (3 Ne. 1:13).
To me it is significant that in the 19th-century revelations of the Doctrine and Covenants that admonition most often comes as an assurance that the Messiah is with us. The Lord says "Be of good cheer, little children; for I am in your midst, and I have not forsaken you" (D&C 61:36); "Be of good cheer, and do not fear, for I the Lord am with you, and will stand by you" (D&C 68:6); and "Be of good cheer, for I will lead you along" (D&C 78:18).
The promise is that the Lord will be with us and lead us, presumably through the Comforter (Jn. 14:16, 26; 15:26). But we often misunderstand what comforter means. We take it to mean one who consoles us and, of course, in modern English that is its primary meaning. But the older meaning of the word is more closely associated with its roots, which mean "strength with." A comforter is someone who gives you strength. That's why comforter is a good translation of the Greek word used in John, parakletos, for the latter means "one who comes to the aid of another." We are promised that God will be with us through the Holy Spirit, and we are told that we should have courage in the face of trials because we are assured of his divine presence with us in those trials.