As you might be able to tell from the title, mountains are kind of a big deal in the novel. There's lots of climbing and falling imagery, all having to do with the journey towards holiness. In simplistic terms, up means good and down means bad: it's a spiritual version of Chutes and Ladders.
One of the times the whole "falling and climbing" symbolism comes into play is when John heads for Central Park to decide how to spend his birthday cash. He finds his favorite hill, and from the top of it feels like:
… a giant who might crumble this city with his anger; he felt like a tyrant who might crush this city beneath his heel; he felt like a long-awaited conqueror at whose feet flowers would be strewn and before whom multitudes cried, Hosanna! (1.1.116).
Now this moment adds a little depth the to the easiness of the "up is good, down is bad" symbolism. Because, as we can see, at the top (after the climb up) is a feeling of raw power. This adds a little nuance to this Good v. Evil, Salvation v. Temptation game: there's a pretty dandy reward at the top of the mountain, which might make the climb less about goodness than about, well, getting power.
But we'll come back to all that. Let's check out some descents down the mountainside first. When John leaves that peak in Central Park, he's running down into the city of sin (woohoo!), and he knows the danger. He relates it to the descent: "'I can climb back up. If it's wrong, I can always climb back up'" (1.1.119).
Later on, John's mother, Elizabeth, feels the same metaphor. When she starts falling for her boyfriend, Richard, she relates it to being "on the edge of a steep place: and down she rushed, on the descent uncaring, into the dreadful sea" (2.3.72). The sin of premarital sex is the descent she's talking about, as well as the sensation of "falling" in love.
After Richard's death, getting back to good, sin-free standing in the community isn't so easy. "She would have to climb back up alone […]" (2.3.85). For Elizabeth, climbing back up is harder because this time she's carting a chubby little baby.
The image of the mountain as holy comes back to John at the end of the novel. In a conversation with Elisha, he seems to know that it won't be easy to just stay put in his new, holy state. He'll have to live in the world, with his crazy father and brother, and with his desire for other boys (see John's "Character Analysis" for more on that) and his uphill climb will continue
He confirms this, asking Elisha, "It's a long way," John said slowly, "ain't it? It's a hard way. It's uphill all the way" (3.1.232). John knows how difficult his holy-man life is going to be.
And so does Elisha: "You remember Jesus," Elisha said. "You keep your mind on Jesus. He went that way—up the steep side of the mountain—and He was carrying the cross, and didn't nobody help Him. He went that way for us. He carried that cross for us" (3.1.233).
The image of the novel melds with an image from the Bible, the mountain of Calvary where Jesus was crucified. It represents sacrifice, which is exactly what John is preparing for.
But even as John is preparing for sacrifice, he feels like he's already won a battle in the war against Gabriel. Gabriel has long had the power in the Grimes household—he's the freaking paterfamilias—but after John is saved, the power dynamic at home shifts suddenly. John and Gabriel are now both grown men, on slightly more equal footing. And there's definitely a suggestion that John will hold the lion's share of the power sooner rather than later.
So yeah, a literary genius like Baldwin isn't going to leave us with just a symbol that states, "Herpity derp durr—up means good and down means bad." He's way too smart and nuanced for that. Up means good… but it also means ascending toward power, which ain't so great. Down means sin… but it also means relinquishing power, which isn't so bad.