Why does Goodman Brown say, "Of all nights in the year, this one night must I tarry away from thee," and why does he feel he must journey "twixt...

Young Goodman Brown feels that he must test the power of good, so he attends the Black Mass one night.  This, in fact, is the night on which he must "tarry away" from his new bride, Faith.  Certainly, the symbolism of her name cannot be lost upon the reader of Hawthorne's story.

This statement also underscores the theme of Guilt vs. Innoncence and Good vs. Evil in "Young Goodman Brown."  For, Goodman in his religious complacency believes that he can traverse the forest with the devil, encounter hypocrites such as Goody Cloyse and Deacon Gookin (real characters involved in the Salem Witchcraft Trials of 1692) and return spiritually unscathed.

However, while his wife takes the "leap of faith" and welcomes Goodman home the next day, Goodman proves his statement that he must "tarry away from you" to be ironic.  For, after his traveling between good and evil,--"twixt now and sunrise"--he no longer believes in his wife's innocence or the prevailing innocence of the Puritans in his community; he has, indeed, moved away from his faith.  Henceforth, he now believes what the "sable form" of the devil has told him:  "Evil is the nature of mankind."

A stern, a sad, a darkly meditative, a distrustful, if not a desperate man did he become from the night of that fearful dream.

Critics often view this shrinking "from the bosom of Faith" of Young Goodman Brown as an indictment by Hawthorne of the unredemptive nature of Puritanism, a belief in the evil nature of human beings because of original sin, a sin that damns most people for eternity.