TULSA, Oklahoma - Pope Benedict XVI stunned the world with word he's resigning at the end of the month.

That makes him the first Pope to step down in 600 years.

The 85-year-old Pontiff said his duties require strength of mind and body and he simply isn't strong enough anymore.

Doctors recently advised the Pope not to take any more trans-Atlantic trips.

The Pope's decision caught many Catholics off-guard, including many leaders of the faith.

Saddened, shocked, and stunned: those are just some of the reactions from people attending Mass, Monday, in Tulsa.

By the time parishioners went to Mass at Tulsa's Holy Family Cathedral, most had heard the news that has stunned church members around the globe.

"I'm disappointed that he's not there, because I thought he was a great leader," said Richard Hays.

Bishop Edward Slattery leads the Diocese of Tulsa, which is comprised of 31 counties in the eastern part of Oklahoma. Slattery found out about the Pope's decision early Monday morning.

"Well, my first reaction was sadness and shock, because I have such high regard for this Pope," Bishop Slattery said.

He said Bishops around the world feel close to Benedict, who came to the Papacy with an image of a cold theologian, but quickly convinced the world otherwise.

"Find him to be warm a true churchman, and he's fearless," Slattery said.

The Bishop said many church leaders were delighted when Benedict was elected Pope, and assumed he would die in office.

Slattery said Benedict will be remembered for his fearless evangelism, but also his charity.

"Benedict XVI saw the Christian message as basically a message of forgiveness," he said.

The 85-year-old Pope emphasized that carrying out the duties of leading more than a billion Roman Catholics worldwide requires "both strength of mind and body."

"He probably feels he's not doing justice to the job, or won't be able to," Slattery said.

But many Catholic faithful believe, while serving as Pope, Benedict has done a tremendous job.

"Get Catholics to understand their faith better, know their faith, be active in their churches, and I think he's done a wonderful job," said Terri Beam.

Both church leaders and parishioners say, while the Pope will be different, the message will remain the same.

"The head of the church is still Jesus Christ, so it's just a temporary thing," Hays said.

Bishop Slattery said he does not think there is a frontrunner for the Papacy, right now, and that the next Pope will likely be a surprise, possibly coming from a part of the world that has never produced a Pope before, like Asia, Latin America or Africa.