Terabithia is created by Leslie and Jess's conviction that they require a space of their own, a magic space where no one will bother them and where they can hang out in peace. On an early day in their friendship, Leslie tells Jess her idea:
"We need a place," she said, "just for us. It would be so secret that we would never tell anyone in the whole world about it." Jess came swinging back and dragged his feet to stop. She lowered her voice almost to a whisper. "It might be a whole secret country," she continued, "and you and I would be the rulers of it." (4.98)
Their first ideas about Terabithia come from the Narnia books (by C.S. Lewis), which Leslie lends Jess. They build a castle in their own imagined land, declare themselves rulers, and create customs and ceremonies to observe. In Terabithia, they fight imaginary battles, become victorious, and conduct celebratory feasts. We don't learn much about what Terabithia looks like physically. We know you have to get there by crossing the creek in between their farms and the forest: the place where they build their castle is "where the dogwood and redbud played hide and seek between the oaks and evergreens, and the sun flung itself in golden streams through the trees to splash warmly at their feet" (4.105). To properly "see" Terabithia, we just need to use our imaginations, just like Jess and Leslie do. (For more on Terabithia's significance, see "What's Up With The Title?")
For most of the book, Jess and Leslie get to Terabithia "by swinging across [the creek] on this enchanted rope" (4.102). In a series of tragic coincidences, the rope gives out when Leslie is swinging on it by herself and when the creek is completely full. The danger Jess felt but couldn't express comes suddenly, horribly true. It's nobody's fault, but it can't be undone. After the tragedy, Jess wonders if their magic land is still magical, "If it was still Terabithia. If it could be entered across a branch instead of swung into" (13.5). It takes him a while to figure out that it is, and it can – and that not going back to Terabithia would be a disservice to Leslie. Yet, after his close call with May Belle, Jess realizes there has to be a better way, a new way, into Terabithia:
"The next day after school, Jess went down and got the lumber he needed, carrying it a couple of boards at a time to the creek bank. He put the two longest pieces across at the narrow place upstream from the crab apple tree, and when he was sure they were as firm and even as he could make them, he began to nail on the crosspieces." (13.69)
As if Leslie's memory wouldn't be present each time Jess went to Terabithia, he's ensured a memorial to his dear friend by using lumber from her parents' house to build the actual bridge. The bridge Jess makes doesn't look like much: although Jess knows it's "the great bridge into Terabithia," he understands it "might look to someone with no magic in him like a few planks across a nearly dry gully" (13.82). But it's another way of bringing the magic Leslie saw in Terabithia to the outside world where he can share it with others. (For more on the bridge's significance, see "What's Up With the Title?" and "What's Up With the Ending?")
None of the characters in the book are very religious, but some of them still attend church. Leslie's family doesn't go at all, and Jess's only makes it there once a year. For the Aarons family, church seems to be as much about presenting a good front to the community as it is about actually believing. We hear about belief systems from the very young May Belle, who insists to Leslie that without a belief in the Bible, "God'll damn you to hell when you die" (8.72). Leslie, with her imaginative, secular mindset, disagrees. This is one of the things that worries Jess when Leslie dies, and his father reassures him: "God ain't gonna send any little girls to hell" (12.34). Yet some people see these ideas as "anti-religious sentiments" instead of comforting ideas (source). For more on how the topic of religion is touched on in potential censorship, this website and Shmoop's "Intro."