In The Scarlet Letter, Reverend Arthur Dimmesdale is considered a very honorable person by almost everyone in the Puritan town. Practically no one would believe that he would have the ability to do any evil, much less the sin of adultery. On the contrary, Dimmesdale feels that he is a terrible person for committing this sin and not admitting it to the townspeople. This fact affects him greatly yet unexpectedly increases his popularity by inspiring him to come about with more intensifying sermons.
Adversely to the common opinion of the townspeople, Reverend Arthur Dimmesdale is not very honorable and does not deserve any praise whatsoever. In fact, he is a coward. Dimmesdale is not courageous enough to tell the town that he was the one who committed adultery with Hester and the one who deserved to stand in the scaffold with Hester and Pearl while they are being punished openly by the townspeople's stares and whispers. This reverend is only able to stand in the guilty spotlight when the sun is down and he is covered by the veil of darkness, the veil of evil. Only under this cloak, is he able to announce his sin and stand on the scaffold. However, Pearl, his adulterous daughter, refuses this confession because he declines her offer to stand with her and her mother the very next day at noon, in front of everyone and in broad daylight.
Besides being a milksop, Arthur Dimmesdale is also a masochist. He has become a masochist because of his lack of audacity to own up to his sin. Following the example of Hester's scarlet letter, Dimmesdale decides to make the "scarlet letter" appear not just in his heart, but on his chest as well. Being a masochist seems to accomplish this form of expressing his guilt. Whipping himself, Dimmesdale soon creates a mark on his chest that he could consider as his "scarlet letter." Nevertheless, this "scarlet letter" is always hidden underneath a shirt.
In addition to hurting himself this way, Dimmesdale also fasts and keeps fasting for long stretches of time. Because of his fasting and the mark he created on his chest, the reverend begins to have hallucinations. These hallucinations are usually about his parents turning away from him because of the dreadful sin that he committed. They also include Hester (with Pearl) pointing to the scarlet letter on her chest and then pointing back at Dimmesdale's chest, where his "scarlet letter" is located.
Furthermore, Arthur Dimmesdale seems to believe that the injuring of his body is supposed to purify himself from the sin he committed, without the punishment of other townspeople muttering about him and using him as an example of what not to do. Unfortunately for him, Dimmesdale does not receive any comfort from trying to cleanse his soul by marring himself. He continues trying to decrease this feeling of guilt by giving great sermons and telling the townspeople that he is "altogether vile, a viler companion of the vilest, the worst of sinners, and abomination, a thing of unimaginable inequity" but no one believes him. In the end, he stands on the scaffold and "declares" his sinfulness in the middle of the night, when no one will take any notice of him. This gives him temporary relief from his guilty conscience.
As you can see, Reverend Arthur Dimmesdale is a coward, a masochist, and basically a hopeless man who keeps lying to himself that he can purge his soul of the heinous sin without openly revealing it. He will never be able to be rid of his guilty conscience unless he confesses to the townspeople and most importantly, to God.