Dimmesdale's guilt seems to be making him somewhat less discerning. Chillingworth had chosen Dimmesdale to be his spiritual guide, and it was at about this time that "the health of Mr. Dimmesdale had evidently begun to fail." Although people in the town were, at first, very hopeful that Chillingworth would be able to help their poor minister, they soon come to believe that "the Reverend Arthur Dimmesdale [...] was haunted either by Satan himself, or Satan's emissary, in the guise of old Roger Chillingworth." Dimmesdale had allowed himself to be matched up Chillingworth as roommates, and -- even in very close quarters -- he seems unable to realize that the doctor is trying to "[dig] into [his] heart, like a miner searching for gold."
Further, the narrator says,
Mr. Dimmesdale would perhaps have seen this individual's character more perfectly, if a certain morbidness, to which sick hearts are liable, had not rendered him suspicious of all mankind. Trusting no man as his friend, he could not recognize his enemy when the latter actually appeared.
Thus, we can understand that Dimmesdale's "sick heart" is the result of his guilt, and this guilt has made him distrustful of everyone. Because he is constantly wary of all, he is unable to pick up on the fact that he ought to be warier of one person than anyone else. Therefore, his discernment has most definitely decreased as a result of his guilt.