This interview, reprinted with permission of National Catholic Register, originally appeared on Beliefnet in 2003.

Father Brian Kolodiejchuk, the Missionary of Charity priest who is advancing Mother Teresa's sainthood cause, was surprised. Like much of the world, he thought the story of Mother Teresa was summed up by her simple faith. But the work he did--work that came to fruition in Oct.19, 2003 when Pope John Paul II beatified Mother--revealed much more. He told Rome correspondent Father Raymond J. de Souza about it.

What did you learn about Mother Teresa in the course of preparing her cause?

Even though she was a public person, she managed to keep her interior life private. She hardly ever said anything about her personal life. She wasn't interested in a biography, as she did not see that as important. Her focus was always that everything she did was God's work. She would always say that to anyone who spoke of her success: "It's God's work."

People saw her holiness - it was evident - but now we realize that her simplicity hid a real profundity. We are just beginning to scratch the surface of the more profound aspects of her soul. I think we will see that she is one of the greater saints, but time will tell as we begin to understand more of her.

What is the principal message of Mother Teresa's life?

A reminder of how much God loves human beings, expressed in those key words from the cross: I thirst. Thirst gives an idea of how intense is the desire of God for souls.

Indian Prime Minister Indira Gandhi presented her with an award one time, and she said that Mother Teresa reminds us of the simple importance of love.

When leaders would ask Mother why she wanted to come to their country or city, she would say that she wanted to make the love of God present. Love becomes concrete in compassion. As the sisters began to expand in the West and Mother would travel, she realized that the greater poverty is to be unloved, unwanted, rejected and lonely - all of which is very common in the West. Nevertheless, Jesus loves you - he thirsts for you.

What did you learn about Mother Teresa that you did not know before?

There were three major new things.

First, how very far advanced Mother was in the spiritual life by the time of her inspiration to found the Missionaries of Charity in 1946. Evidence of this was the private vow she made in 1942 not to refuse God anything - which, if you think about it, is quite a daring thing to do. Love unites, and this vow showed how united her will was to the will of God.

Second, we learned more about what happened when she was inspired to found the Missionaries of Charity. She never spoke about what she experienced exactly, but fortunately she wrote it down in a letter to the archbishop of Calcutta. She didn't write about what happened on Sept. 10 - what we call "inspiration day," when she received what she termed her "call within a call"- but she describes what happened in the days afterward during her retreat. She writes about her locutions in which Jesus asks her, "Wilt thou refuse?"

So we see that her vow was being put to the test.

Third, that soon after the inspiration there was a real, close, intense union with Jesus in 1946 and 1947. But then, once the work started, that sense of union goes away, and for the rest of the time she proceeds by pure faith and pure love. This "darkness" as she called it, was a classic experience of the "dark night" that the saints speak about.Yet there was also a tremendous sense of longing for the union that she was not experiencing (read more). This was for us a new way of understanding the "I thirst" of Jesus - and often Mother would speak of a "painful thirst." Mother was sharing in the longing and sufferings of her beloved.

So Mother was not only sharing in the physical poverty of the poor but also the sufferings of Jesus - his longing for union, as expressed in Gethsemane and on the cross.

And at the same time as all this was going on, she used to say that she wanted to be an "apostle of joy."

How long did this period of darkness last?

Till the end. Fifty years. This seems to me the most heroic thing of her spiritual life.

From what sources did you discover these new things? Did she keep a diary?

She did not have a diary. It was from the letters she wrote to her spiritual directors over a long period of time.

I don't think she wanted those letters to be kept, but thankfully the Jesuits [who served as her spiritual directors] had enough sense to realize that those letters were going to be important.