Goodman Brown begins “doubting whether there really was a heaven above him”. Why? What events cause him to begin to doubt?

In the story, Goodman Brown starts to doubt the reality of Heaven after witnessing discrepancies in the actions of well-respected church leaders.

At the beginning of the story, we see that our protagonist approaches his 'evil purpose' with trepidation. He laments that he has gone as far as he has on this treacherous errand.

"My father never went into the woods on such an errand, nor his father before him. We have been a race of honest men and good Christians, since the days of the martyrs. And shall I be the first of the name of Brown, that ever took this path and kept--"

However, the Devil shocks Goodman Brown by telling him that he is well acquainted with his father and grandfather. After all, Goodman Brown's father participated in the Salem persecution of the Quakers, while his grandfather participated in torching Indian villages during King Phillip's War.

The Devil continues to torment Goodman Brown by stating that various well-respected church deacons, religious elders, town leaders, judges, and even the Governor of Massachusetts count him among their friends. Goodman Brown is deeply disturbed when he hears this, but he tries to make the excuse that, since he doesn't have any real interactions with these leaders, he cannot be expected to make a judgment on their motives. However, he balks at possibly meeting his priest or minister while out on his evil errand.

At this, the Devil laughs uproariously. We see why when both travelers come across Dame Goody Cloyse. This is the woman who has taught Goodman Brown his catechism. Goodman Brown is deeply dismayed when he hears this supposedly devout woman boast about being anointed with witches' herbs and 'the fat of a newborn babe.' When Goody Cloyse subsequently complains about another witch possibly appropriating her broom for her own use, Goodman Brown has heard quite enough. He loudly asserts that he will not take another step with his devilish companion.

The Devil, however, advises Goodman Brown to rest for a bit, stressing that he may change his mind as he contemplates his situation. While being left alone, Goodman Brown sees yet another scene which causes him to doubt the reality of God and heaven. As hoofs thunder towards his direction, he spies his minister and Deacon Gookin talking animatedly about their macabre errand. Both are on their way to the satanic congregation of witches and devils. What's more, Deacon Gookin exclaims excitedly about a 'goodly young woman to be taken into communion' at the convocation of sinners.

At this, Goodman Brown feels 'faint and overburthened with the heavy sickness of his heart.' He begins to doubt his senses and his own prevailing faith in everything he has ever been taught. If every devout religious leader he knows is really evil at heart, what hope does he have that heaven is real? Goodman Brown is so shaken that he finds it difficult to continue on his journey.