Section 29 summarizes the plan of salvation, which can be summarized with verse 5:
Lift up your hearts and be glad, for I am in your midst, and am your advocate with the Father; and it is his good will to give you the kingdom.
(Doctrine and Covenants 29:5)
This is a fascinating contrast to the "tribulation and desolation" described in verses 8-21. These verses describe things that most people would rather not focus on, such as "there shall be weeping and wailing among the hosts of men," and "I the Lord God will send forth flies upon the face of the earth, which shall take hold of the inhabitants thereof, and shall eat their flesh, and shall cause maggots to come in upon them; And their tongues shall be stayed that they shall not utter against me; and their flesh shall fall from off their bones, and their eyes from their sockets; And it shall come to pass that the beasts of the forest and the fowls of the air shall devour them up." (Doctrine and Covenants 29:18–20)
One answer is found in verse 34.
Wherefore, verily I say unto you that all things unto me are spiritual, and not at any time have I given unto you a law which was temporal; neither any man, nor the children of men; neither Adam, your father, whom I created.
(Doctrine and Covenants 29:34)
Section 29 reaffirms the literal creation of Adam and that he was cast out from the Garden of Eden. t also emphasizes that we cannot be agents under ourselves without the temptations of the devil (verse 39), thereby reiterating the reality of the devil.
And it must needs be that the devil should tempt the children of men, or they could not be agents unto themselves; for if they never should have bitter they could not know the sweet—
(Doctrine and Covenants 29:39)
The combination of "lift up your hearts" with "be glad" appears only in the D&C and PofGP (Moses), but both elements are found in the scriptures.