But Mohammed Shafiq, chief executive of the Ramadhan Foundation, offered a different take. He referenced a 2006 speech Benedict made in which he quoted from 14th century Byzantine emperor Manuel II Paleologus saying the Muslim Prophet Mohammed brought into the world "things only evil and inhuman." These remarks spurred protests by Muslims worldwide and an apology from the pope.

"This sadly meant the hard work of his predecessor Pope John Paul II was tarnished and required extensive work to rebuild ties between Christianity and Islam," Shafiq said. "That is something he has tried to do over the past eight years, and we do wish it could have started better than it did."

In his tone, demeanor and actions, Benedict was notably different from his predecessor. Where John Paul wowed crowds around the world with his mastery of numerous languages, Benedict's influence will be felt through his writings, part of his training as a college professor, Gallagher said.

Allen called Benedict a "great teaching pope."

Praised, criticized for actions regarding sex abuse

Benedict became pope at the height of the molestation scandal involving Catholic priests, with complaints of sexual abuse and lawsuits over the issue tearing at the church and threatening its moral standing around the world.

In 2008, he acknowledged "the shame which we have all felt" over abuse reports and said those responsible for the "evils" should face justice. Two years later in 2010 -- the same year that he issued new rules aimed at stopping abuse -- he said abusive priests "disfigured their ministry" and brought "profound shame and regret" on the church.

The new rules included allowing church prosecution of suspected molesters for 20 years after the incidents occurred, up from 10 years previously. The rules also made it a church crime to download child pornography and allowed the pope to remove a priest without a formal Vatican trial.

"No one did more to successfully address the problem of priestly sexual abuse than Joseph Ratzinger," Donohue said.

But others -- such as Barbara Blaine, president of Survivors Network of those Abused by Priests -- say the opposite is true.

"I would hate for him to be remembered as someone who did the right thing because from our perspective, Pope Benedict's record has been abysmal," Blaine said.

In 2010, The New York Times reported that church officials, including Ratzinger, had failed to act in the case of a Wisconsin priest accused of molesting up to 200 boys. The Times reported that church officials stopped proceedings against the priest after he wrote Ratzinger, who was at the time the cardinal in charge of the group that oversees Catholic Church doctrine.

Ratzinger never answered the letter, according to the Times, and church officials have said he had no knowledge of the situation. But a lawyer who obtained internal church paperwork said at the time that it "shows a direct line from the victims through the bishops and directly to the man who is now pope."

Also in 2010, the Times reported that the future pope -- while serving as the archbishop in Munich -- had been copied on a memo informing him that a priest accused of molesting children was being returned to pastoral work. At the time, a spokesman for the archdiocese said Ratzinger received hundreds of memos a year, and it was highly unlikely that he had read it.

Victims' groups are pressing the International Criminal Court to prosecute Benedict in the sex abuse scandal and say the resignation won't change that, according to Pam Spees, of the public policy law firm Center for Constitutional Rights, which is helping SNAP pursue the case.

In a statement issued Monday, Blaine said the church should choose a new pope dedicated to preventing sexual abuse by priests.

"For the church to truly embody the spiritual teachings of Jesus Christ, it must be led by a pontiff who demands transparency, exposes child-molesting clerics, punishes wrongdoers and enablers, cooperates with law enforcement and makes true amends to those who were hurt so greatly by Catholic priests, employees and volunteers," she said.

For others, the next pope must be someone who not only can satisfactorily address this scandal and other issues, but he also must be able to speak to all Catholics and others -- especially younger people -- in a way that resonates with them.

"I know that my grandparents think of the whole church a little bit differently than my generation does," said Kaleigh Forst, a student, in New York City. "I feel like we could use somebody maybe a little younger, that has a new perspective."

World, Catholic leaders express surprise, admiration

Whatever his past, few expected Benedict to announce he was stepping down, including world and Catholic leaders, who voiced admiration for his decision and the man himself.

U.S. Vice President Joe Biden, himself a Catholic, said he thought the pope set "an incredibly high standard" by understanding his own limitations and how they might affect the church.

"The decision reinforces for me as a practicing Catholic that this is a man of great integrity and looking out for what he believes is in the best interest of our church," Biden said. "I admire him for it."

British Prime Minister David Cameron said Benedict "will be missed as a spiritual leader to millions." Cameron's Irish counterpart, Enda Kenny, praised Benedict for decades of leadership and service, as well as his decision to resign.

"It reflects his profound sense of duty to the Church, and also his deep appreciation of the unique pressures of spiritual leadership in the modern world," Kenny said in a prepared statement.

Archbishop Vincent Nichols, the president of the Catholic Bishops' Conference of England and Wales, said the decision "shocked and surprised everyone."

"Yet, on reflection, I am sure that many will recognize it to be a decision of great courage and characteristic clarity of mind and action," he said.

Archbishop Timothy Dolan, president of the U.S. Conference of Bishops, said he was startled, and sad, to see Benedict resign.

"It's like watching your own Dad get old and admit he's not up to all the duties that being the head of a family involves," Dolan said. "And there's a somberness, there's a sadness there."