I agree that good deeds are good, the issue is do they save or are they a fruit of your salvation.
The unsaved can and do , do works men call good. All creation is ruled by God, He is the God of the unsaved, even if they do not realize it.. He uses them for His purposes and to achieve His ends .
Scripture says "For all that is not of faith is sin.", Jesus called works of the unsaved done in His name "inequity" .
Then the next question is, "Where does their goodness come from?"
... the issue is do they save or are they a fruit of your salvation.
Well that is AN issue, it is not, however the issue raised by the challenges you made in the direction of Mother Teresa, or I don't see how it is.
The question was about how to understand her remark about if people become better Muslims, Orthodox Presbyterians, whatever, and, in particular the clause, "... then there is something else growing there."
Something may be the most important good to a particular person at a particular time, but that does not mean it is the only good. So when Mother Teresa says what you quoted, it is not clear she is talking about a saving good. Consequently it is not clear that saving goods are properly relevant to a discussion of her work and words.
Another problem with this kind of discontinuous view of the universe is that the same people who take their kids to services and Sunday school and VBS talk as if there were no value in any work undertaken to sort of predispose someone to hear the Gospel or to, as it were prepare the ground -- is it clear to you, and if so what are your sources, that Mother Teresa started her mission to 'convert' the people whe took from the streets? It might not save her or anyone else, but is it a bad thing to provide a little comparative comfort to someone who is dying -- even if the comfort does not 'save' them? Is it inappropriate for churches to, say, feed the hungry? Why or why not?