People filled the pews to pray for the pontiff's health, and for cardinals to make a wise choice in his successor.
"He has served so diligently and so intelligently and so wonderfully. We are so grateful for his guidance and his leadership, and his humility to say, ‘I can't do this anymore,'" said Barbara Johansen.
Atlanta Archbishop Wilton Gregory led the special Mass of Thanksgiving.
"Today's resignation, for me, was not simply of a figure who had a title and had an authority. It was the resignation of someone that I knew and loved and respected as a friend," Gregory said.
Catholic youth leaders called the papal transition an opportunity to connect with the next generation to light a spark of faith.
"It's kind of like losing your poppa today, so it's been an emotional day but it's been a day to pray in thanksgiving and the young church is excited and alive and vibrant," said Stephen Lenehan, the youth ministry director.
Gregory will be covering the process of finding a new pope as a consultant for an international broadcast company.
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