How do you know that? Did she confess to you after she died?
She questioned the God we created, and rightfully so. Again, it's easy for fat cats who have it made to believe, but living in the ghettos of India and the reality of life there would test anyone's faith.
If she doubted the fairytale that's understandable, for anyone living there would. Despite that, she persevered in her work of mercy. If she stopped believing in God and salvation, then her perseverance is that much more commendable, because she did not expect any rewards, but did what she did for love's sake, and love's sake only.
She did not sit in her comfy air conditioned homes, or go to churches all dressedup and perfumed, or sent her donations like the hypocrites do. Like a saint of biblical times, who walked among the lepers, and lived the life of poverty in the hell of Calcutta to comfort the sick and the dying, whose only "fault" is the accident of birth.
What she did is a lot more than 99.99% of self-styled Christians will ever do in the name of Christ. If anyone is saved, Mother Theresa is a good candidate to be among them, for Christ—whom you consider your God—says "blessed are the merciful, for they shall receive mercy."
Rome has concocted a figment of its imagination. That figment did not exist. Instead, she was a tormented and sad woman who was weighted down with fear and doubt.
I can understand why this appeals to you.
It doesn't to most Christians. That's not how they have learned Christ.
Why is it "that much more commendable?"
Paul said "anything not of faith is sin."
So the good offerings of pagans amount to nothing.
Yes, she was a hard worker. No, she was not the model for Christian faith nor practice. Her disbelief does not save her; it condemns her.
We can only hope she changed her mind before she died.