Darkness

Bl. Teresa of Calcutta experienced the "reparatory" dark night of the soul, which is unusual even among the greatest saints. I was once told that The Little Flower also experienced the darkness that is beyond the dark night. According to Zenit:

[Bl. Teresa's] experience of darkness within union is very rare even among the saints because for most, the end is union without it.

Her suffering, then, to use the Dominican theologian Father Reginald Garrigou-Lagrange's term, is reparatory, much more for the sins of others, not purificatory, for her own sins. She is united to Jesus in enough faith and love to share in his experience in the Garden of Gethsemane and on the cross.

Mother Teresa made the comment that the suffering in the Garden was worse than the suffering on the cross. And now we understand where that was coming from, because she understood Jesus' longing for souls.

The important thing is that it is union, and as Carol Zaleski pointed out in her article in First Things, this kind of trial is a new kind of trial. It is a modern kind of experience for the saints over the last 100 years or so, to suffer the feeling that one does not have any faith, and that religion is not true.