Your question refers to the thoughts of Goodman Brown as he takes his leave from his wife and beings to enter the dark and spooky forest, which is so crowded with trees that he reflects there could be any number of people out there hiding behind them, even though it appears Goodman Brown is completely by himself. This leads him to fear that others might be there with him who would do violence unto him, and he says:
"There may be a devilish Indian behind every tree," said Goodman Brown to himself; and he glanced fearfully behind him as he added, "What if the devil himself should be at my very elbow!"
Of course, this is very ironic, because the next moment he turns around a corner and finds a figure who we identify to be the "devil himself" waiting for him as he journeys on his way. Entering the spooky forest and the fear that it inspires in Goodman Brown thus triggers the arrival of the devil as teh father of all evil.