"We had a chance to speak just the two of us, and she said, this is when I was still a seminarian, and she said 'If you will not be holy, please do not be a priest,'" he said.

"Those have been very challenging words through the years, but also very confirming. And she said it with a great smile, not with any consternation or damnation, but with a great, great smile and just a real encouragement," he continued.

Father Kelly first attended the Pontifical North American College, and after being ordained in 1988, he went on to study at the Pontifical Gregorian University, also in Rome.

In addition to his studies, he worked with the Missionaries of Charity, an assignment that led him to have several interactions with Mother Teresa.

On one of those occasions, he recalled, he was the main celebrant at a Mass for about 40 or 50 sisters. All of them sat on the floor, and all of them dressed alike, so it wasn't until halfway through the first reading that he realized Mother Teresa was among the group, he said.

"My mind started jumping ahead to 'Oh gosh, I hope I prepared a good homily,' and then I said 'Oh My God, the reading is the beatitudes!' So, I ended up preaching to Mother Teresa about the beatitudes," he said, laughing.

Another time, Father Kelly said, he asked Mother Teresa to write something in his prayer book. She wrote one of her "popular phrases" in it, "Be only all for Jesus through Mary."

Each time he met with her, he said he noticed that Mother Teresa exuded "a presence and a joy."

"She was accessible. She was very determined, you just got that sense. There was a work to do; there was a mission to do. She loved her sisters, you could see it, the way she interacted with the other nuns, she just loved them, and they really did look at her like a mother," he said.