Final answer:
William Miller, leader of the Millerites, lost credibility due to his failed prediction of the Second Coming of Christ on October 22, 1844, known as the Great Disappointment.
Step-by-step explanation:
The utopian leader William Miller lost credibility by his failure to predict accurately the second coming of Christ. Miller, a Baptist convert and editor of the Advent Herald, led a millennial group known as the Millerites. They believed that the Second Advent of Jesus was imminent and proclaimed that October 22, 1844, would be the date of the Second Coming. When Jesus did not appear on that date, the event became known as the Great Disappointment, leading to the disintegration of the movement.