And God had a lot of things to say to those doubters.
I have a question: after reading your homepage and thinking of what the point is that you may be making about Mother Theresa, is there any way for a person to know whether or not they are among the elect?
Yeah.
A bruised reed he will not break,
and a faintly burning wick he will not quench;
he will faithfully bring forth justice.
-- Isa. 42:3 [ESV]
Doubt and faith are not antithetical. The opposite of faith is antipathy or hostility, not doubt. It takes incredible resilience of faith to hold on and believe when every fiber of your being cries out how screwed up this world is, and seeks to indict God. But, as in Ps. 73, we may be foolish, myopic, and short-sighted, but God upholds us even through our doubts.
Heck - if doubt were some sort of unforgivable sin, than a large number of the Psalms - which were written in raw doubt - would be sinful, and Christ's cry of "My God, my God, why have you forsaken me!?" would be sinful.
The interesting story in Mother Theresa is not that she struggled with doubting God. Frankly, any human who didn't struggle with doubt after seeing what she saw on the streets of Calcutta would be some sort of unfeeling sociopath. Far more intriguing - and uplifting - is how Mother Theresa clung onto her faith in the midst of her doubt, and continued the acts of mercy she felt Christ called her to. Despite deep doubt and despair gnawing at her from within, she pressed on without wavering. That's simply amazing.
Now, as regards the orthodoxy of certain statements of Mother Teresa, I take no position. Too many of those statements are quoted by people with agendas unfriendly to Mother Theresa, wholly divorced from their context. Furthermore, wasn't she Macedonian? Did she speak English well, if at all?