"Thanks for allowing me to share in your miracle," Obama told the nearly 500 graduates of the College of Arts and Sciences and the Graduate School seated at the New Orleans Arena on Saturday.
Norman C. Francis, Xavier's president, said he never doubted that the university would hold graduation ceremonies this year.
"I said shortly after the storm that people would either call me stupid and crazy or an aggressive visionary," Francis said. "I guess it's the latter."
Xavier, the country's only historically black and Roman Catholic college, reopened in January. More than 120 students graduated from the College of Pharmacy in May, and the fall semester will begin Sept. 8 with about 2,800 students.
Obama, a black first-term senator from Illinois who is a rising star in the Democratic Party, noted that most commencement speakers tell graduates what to expect when they enter "the real world."
"But this is different," he said, adding that most in the room have already experienced the "real" world since Katrina.
The senator said he visited New Orleans last month and saw the "pictures of your campus after the storm — submerged classrooms and dorm rooms where books remained open as you left them."
He recalled hearing the stories of 400 students trapped on the roof of a building along with a handwritten sign that read 'Help us' after flooding blocked their escape from the storm-ravaged city.
He said he could give advice about overcoming challenges or about courage and perseverance but "you could probably teach the rest of us" about those things.
"Yours has been quite an education, an education in humanity brought by a force of nature," Obama said.
However, he said, those types of lessons can be unlearned.
"Time can heal and cloud a memory," he said. "But it's your responsibility to remember what happened in New Orleans and make it a part of who you are. Katrina might be the most dramatic test you take but it won't be the last."
Obama said the graduates would be forced to choose a path — one of detachment and indifference or one of involvement.
"The easiest thing is to do nothing at all. Turn off the TV, put down the newspaper and go about your busy lives. Remain detached and indifferent," he said. "But, if you choose to remember what happens when responsibilities are ignored and the buck is passed. ... That asks more of you. Not only to pursue your own individual dreams but also to perfect our collective dream as a nation."
Obama encouraged the graduates to "make this a nation where we are no longer unprepared to meet the challenges of time. Make this a nation worthy of the sacrifices of so many of our citizens. Take the second path.
"Katrina's not the end of tough times for New Orleans or you," he said.