I think perhaps you may have misunderstood the nature of the "crisis" that Mother Teresa expressed in her letters. The excerpts printed in the article do not reflect a doubt of the gospel or God. Consider:
In correspondents to her spiritual confidants, Mother Teresa laments on the “dryness,” “darkness,” “loneliness,” and “torture” she suffers with her inability to feel God’s presence. . . . . Another letter in 1956 read: “Such deep longing for God – and…repulsed – empty – no faith – no love – no zeal. – [The saving of] Souls holds no attraction – Heaven means nothing – pray for me please that I keep smiling at Him in spite of everything.”
I see no doubting of God, but rather a deep personal depression. The lesson could be that even saints get depressed.
And why not? Jesus Christ, the greatest of all, suffered in body and soul. As Isaiah prophesied of the Messiah:
3 He is despised and rejected of men; a man of sorrows, and acquainted with grief: and we hid as it were our faces from him; he was despised, and we esteemed him not.
4 ¶ Surely he hath borne our griefs, and carried our sorrows: yet we did esteem him stricken, smitten of God, and afflicted.
5 But he was wounded for our transgressions, he was bruised for our iniquities: the chastisement of our peace was upon him; and with his stripes we are healed.
6 All we like sheep have gone astray; we have turned every one to his own way; and the LORD hath laid on him the iniquity of us all.
7 He was oppressed, and he was afflicted, yet he opened not his mouth: he is brought as a lamb to the slaughter, and as a sheep before her shearers is dumb, so he openeth not his mouth.
8 He was taken from prison and from judgment: and who shall declare his generation? for he was cut off out of the land of the living: for the transgression of my people was he stricken.
9 And he made his grave with the wicked, and with the rich in his death; because he had done no violence, neither was any deceit in his mouth.
10 ¶ Yet it pleased the LORD to bruise him; he hath put him to grief: when thou shalt make his soul an offering for sin, he shall see his seed, he shall prolong his days, and the pleasure of the LORD shall prosper in his hand.
11 He shall see of the travail of his soul, and shall be satisfied: by his knowledge shall my righteous servant justify many; for he shall bear their iniquities. (Isaiah 53:3–11)
Is that message "great for evangelism"? Apparently Jesus thought so:
23 ¶ And he said to them all, If any man will come after me, let him deny himself, and take up his cross daily, and follow me. (Luke 9: 23; see also Matt. 16: 24, Mark 8: 34, Mark 10: 21)27 And whosoever doth not bear his cross, and come after me, cannot be my disciple. (Luke 14: 27; see also Matt. 10: 38, )
. . Another letter in 1956 read: “Such deep longing for God – and…repulsed – empty – no faith – no love – no zeal. – [The saving of] Souls holds no attraction – Heaven means nothing – pray for me please that I keep smiling at Him in spite of everything.”
You can't equate that to ANYTHING Jesus said.
I do not believe Teresa was a true believer since she thought everyone who believed in some religion would also go to heaven. She was quite unorthodox. Maybe those idols in her dwelling were an obstacle. It is strange how many people think she must be a true Christian because she did so much charity work and lived in chosen poverty. But does that save anyone?