Pope Benedict XVI has acknowledged Vatican mistakes over a Holocaust-denying bishop and his efforts to reach out to ultraconservatives, saying in a highly unusual critical review that he was saddened that even Catholics attacked him with open hostility. The pope made a personal analysis of the case in a letter to the world's Catholic bishops made public by the Vatican on Thursday, seeking to end one of the most serious crises of his nearly four-year papacy. He said failure to detect the bishop's background by simply consulting the Internet was an "unforeseen mishap" that caused tensions between Christians and Jews and raised questions about his own interest in friendship between the two religions. He said he is saddened that even Catholics who should know better "thought they had to attack me with open hostility." Vatican spokesman the Rev. Federico Lombardi said the letter - released in six languages - was "really unusual and deserving of maximum attention." Benedict acted to limit damage, as he did when he said he was "deeply sorry" over remarks in 2006 about Islam and violence that caused a storm in the Islamic world. The latest crisis began when Benedict lifted the excommunications of four ultraconservative bishops, including British-born Bishop Richard Williamson. Williamson had denied in an interview with Swedish TV broadcast in January that 6 million Jews were killed by the Nazis. He said about 200,000 or 300,000 were murdered and none were gassed. Benedict said Williamson's views were an "unforeseen mishap" that made his efforts of "mercy" toward the excommunicated bishops seem like a repudiation of reconciliation between Christians and Jews. "That this overlapping of two opposing processes took place and momentarily upset peace between Christians and Jews, as well as peace within the church, is something which I can only deeply deplore," he wrote. He thanked "all the more our Jewish friends" for understanding his commitment to friendship. Benedict defended his attempts to bring ultraconservative faithful loyal to the anti-modernization movement of the late Archbishop Marcel Lefebvre back into the church's fold. But he acknowledged that "another mistake, which I deeply regret" was made in not properly explaining his intentions and the limits of the procedure and that some groups had accused him of seeking to "turn back the clock." "That the quiet gesture of extending a hand gave rise to a huge uproar, and this became exactly the opposite of a gesture of reconciliation, is a fact which we must accept," Benedict said. But he said the church cannot be indifferent to a movement that counts 491 priests, 215 seminarians and six seminaries. "Should we casually let them drift farther from the church?" he asked.