What should be the opinion of Muslims regarding Mother Teresa?
The criteria for salvation in Islam are mentioned in Surah Al Asr:
“By (the Token of) Time (through the ages),
Verily Man is in loss,
Except such as have Faith, and do righteous deeds, and (join together) in the mutual teaching of Truth, and of Patience and Constancy.” 
If one wants to have salvation in Islam then all the above four criteria should be fulfilled. If any one of them is missing then one will be in a state of loss in the hereafter. One may have righteous deeds to his//her credit but without faith there is no salvation.
Suppose one has four compulsory subjects to pass in the exam i.e., Math, Physics, Chemistry and Biology; will one be promoted to the next grade, if one fails in just one of the subjects? Even if a person gets full marks in the other three subjects, it will not have any effect on the marks of the subject one has failed in.
So Mother Teresa might have done a lot of work for human welfare but since she died as a mushrik, i.e., while associating partners with God and failed in the test of emaan, her good deeds would not benefit her to achieve salvation in the hereafter.
Allah clearly stated in the Qur’an that He might forgive any sin if He pleases but if one dies as a mushrik without repenting then Allah will not forgive him.
“Allah forgiveth not that partners should be set up with Him; but He forgiveth anything else, to whom He pleaseth; to set up partners with Allah is to devise a sin most heinous indeed.”
Mother Teresa received the reward for her good deeds in this world itself and as such she will have no share in the life hereafter. Hence as per Surah Al Asr she will be in a state of loss in the Aakhirah.
 [Qur’an 103]
 [Qur'an 4:48]< Back to Questions