In the Dhamma, doubt is one of the five hindrances.
Doubt specifically about the Dhamma also causes issues...
Yet, if one has absolutely certainty of their understanding of the Dhamma, and has no doubt about it whatsoever, they may be closing their minds to further advancement...AN 4.184 wrote:"Furthermore, there is the case of the person in doubt & perplexity, who has not arrived at certainty with regard to the True Dhamma. Then he comes down with a serious disease. As he comes down with a serious disease, the thought occurs to him, 'How doubtful & perplexed I am! I have not arrived at any certainty with regard to the True Dhamma!' He grieves & is tormented, weeps, beats his breast, & grows delirious. This, too, is a person who, subject to death, is afraid & in terror of death.
What is the "middle way" between the two extremes of experiencing debilitating skeptical doubt, and ensuring that we don't falsely conclude that "Only this is true; anything else is worthless"?MN 95 wrote:"Bharadvaja, first you went by conviction. Now you speak of unbroken tradition. There are five things that can turn out in two ways in the here-&-now. Which five? Conviction, liking, unbroken tradition, reasoning by analogy, & an agreement through pondering views. These are the five things that can turn out in two ways in the here-&-now. Now some things are firmly held in conviction and yet vain, empty, & false. Some things are not firmly held in conviction, and yet they are genuine, factual, & unmistaken. Some things are well-liked... truly an unbroken tradition... well-reasoned... Some things are well-pondered and yet vain, empty, & false. Some things are not well-pondered, and yet they are genuine, factual, & unmistaken. In these cases it isn't proper for a knowledgeable person who safeguards the truth to come to a definite conclusion, 'Only this is true; anything else is worthless."