Vatican City, Apr 4, 2016 / 03:02 am (CNA/EWTN News).- There are many things about Blessed Mother Teresa of Calcutta that could be called heroic – her tireless service to the world's most rejected and her courageous witness to millions of what it is to live the Gospel, just to name a couple.

But the priest charged with overseeing her path to sainthood said that for him, one thing stands out above all the rest: her experience of spiritual darkness and what she described as feeling totally abandoned by God for the majority of her life.

“The single most heroic thing is exactly her darkness. That pure living, that pure, naked faith,” Fr. Brian Kolodiejchuk, the postulator for Mother Teresa's canonization cause, told CNA in an interview. Fr. Kolodiejchuk is a priest of the Missionaries of Charity Fathers, founded by Mother Teresa in 1989.

By undergoing the depth and duration of the desolation she experienced and doing everything that she did for others in spite of it, “that's really very heroic,” he said.

Pope Francis recently approved the second and last miracle needed in order to declare Mother Teresa a saint, and has set the date of her canonization for Sept. 4, 2016 – the day before her feast day.

Born Agnes Gonxha Bojaxhiu Aug. 26, 1910 in Skopje, in what is now Macedonia, Mother Teresa joined the Sisters of Loretto at the age of 17, but later left after she felt what she called “an order” from God to leave the convent and to live among the poor.

She went on to found several communities of both active and contemplative Missionaries of Charity, which include religious sisters, brothers, and priests.

The first community of active sisters was founded in 1950. An order of active brothers was founded nearly 20 years later in 1968. Then two contemplative orders came, one of women (in 1976) and one of men (in 1979).

In 1989 the Missionaries of Charity Fathers was established, and is a clerical religious institute of diocesan right whose members make promises of poverty, chastity, obedience, and wholehearted and free service to the poorest of the poor.

Additionally, an order of lay missionaries was also founded in 1984, and several movements who organize various works of charity have also been born as part of the Missionaries of Charity spiritual family.

One of the first steps in declaring someone a saint is to determine their heroic virtue. Fr. Kolodiejchuk said that Mother Teresa's entire life was lived heroically, which is clear from what he has seen firsthand and heard from the testimonies of others, even though he himself has only been a part of the Missionaries of Charity family for 20 years.

He said the most heroic aspect of Mother Teresa's life and vocation is the more than 50 years of darkness and abandonment she felt after receiving what she termed “a call within a call” to leave the Sisters of Loretto and found the Missionaries of Charity.

Although the Albanian nun is always seen beaming and smiling brightly in photos, she experienced a profound internal desolation during which she felt silence and rejection from God, who seemed distant.

In a letter to her spiritual director in 1957, Mother Teresa wrote that “I call, I cling, I want, and there is no one to answer. Where I try to raise my thoughts to heaven, there is such convicting emptiness that those very thoughts return like sharp knives and hurt my very soul.”

“Love – the word – it brings nothing. I am told God lives in me – and yet the reality of darkness and coldness and emptiness is so great that nothing touches my soul,” she said.

Mother Teresa had prayed fervently to share in Jesus’ suffering, and many, including her spiritual director, believed her feelings of rejection and abandonment to be a mirror of Christ's own experience of loneliness and desolation during his Passion and death.

Because of the depth and duration of Mother Teresa's spiritual desert, many have hailed her as a great mystic when it comes to topic of spiritual darkness.

Fr. Kolodiejchuk himself said Mother Teresa was “a great mystic, but also very concrete, very down to earth.”

The priest had met Mother Teresa in his early 20s while attending the vows of his sister, who had joined the active branch of the Missionaries of Charity sisters. He joined the order of priests a year later.

A lot of people “think that saints are somewhere in the mystical clouds,” he said, but cautioned that this wasn’t true of Mother Teresa, who was spiritual, but also observant and active in the lives of others.

From the first moment he met her, of Mother Teresa's most distinguishing qualities was “this sense that she really was Mother,” he said, explaining that being a mother was something important to her, and was the only thing she was ever called.

When Mother Teresa was first elected superior general of the Missionaries of Charity, her immediate response after receiving congratulations, he noted, was to say “Oh that means nothing, the title. No, I want to be a mother.”

The nun also placed a heavy emphasis on God’s tenderness, Fr. Kolodiejchuk said, recalling that “tender” was one of her favorite words – even more so than mercy.

“She would talk more about Jesus’ tender love and mercy; his thoughtfulness, his presence, his compassion…So mercy was a word in her vocabulary, but with this quality especially of tenderness.”

“Even in the darkness she still had an intimate sense of God’s tender love for us,” he said, and recited a prayer that Mother Teresa would often teach and have others repeat: “Jesus in my heart, I believe in your tender love for me. I love you.”

The priest said the fact that her canonization is taking place during the Jubilee of Mercy is providential since the core mission of the Missionaries of Charity is to respond to Chapter 25 in the Gospel of Matthew, which lists the works of mercy.

He noted how the day of Mother Teresa’s canonization also marks a special jubilee day for workers and volunteers of mercy.

Given the work the Missionaries of Charity do, “it’s appropriate” that the nun will become a patroness for all who carry out the same type of activities, he said.

Part of the reason Mother Teresa is such a strong example for the world today, Fr. Kolodiejchuk believes, is because “people like to see,” and the work the Missionaries do is something visible that others can easily touch and participate in, no matter what religion they profess.

“Mother was a great believer in that we receive in giving. So there’s something attractive about the work. And then you receive by sharing in it,” he said.

Fr. Kolodiejchuk said the whole Missionaries of Charity family is excited for their foundress’ upcoming canonization, and have already begun to prepare for the big day.

The members of the consecrated orders will have their own spiritual preparations over the next few months, the priest said. Then in September, the days leading up to the canonization in Rome will have a special itinerary, which is still being developed, but will include holy hours and various Masses.

Thousands are expected to come for the event. While it’s difficult to estimate a specific number, Fr. Kolodiejchuk said that between the Missionaries of Charity and those who come for the Jubilee of Workers and Volunteers of Mercy, “it should be a big crowd.”