The cross foreshadows the ending, when Mr. Shiftlet will realize he lost faith.
Foreshadowing is a hint the author makes in the story about what is going to happen in the future.
When Mr. Shiftlet first appears, he seems very Christ-like in his demeanor and attitude. He is a carpenter, he does not care about money, and he seems to care mostly about nature. He resists Mrs. Crawford’s efforts to marry off her daughter. He seems like a humble, good man.
Yet there is foreshadowing that this is not so. First of all, his name implies either that he will make a shift, or that he is shifty—not to be trusted.
He swung both his whole and his short arm up slowly so that they indicated an expanse of sky and his figure formed a crooked cross.
The cross is crooked. We might wonder if that means that the man is crooked, or that his religion is going to become crooked. Depending on how you view the story, either is possible. Shiftlet either was bad all along and pretended to be pious, or he was a religious man and became corrupted. Either way, it is foreshadowed by the crooked cross.