Forgive them anyway.
If you are kind, people may accuse you of selfish, ulterior motives.
Be kind anyway.
If you are successful, you will win some unfaithful friends and some genuine enemies.
If you are honest and sincere people may deceive you.
Be honest and sincere anyway.
What you spend years creating, others could destroy overnight.
If you find serenity and happiness, some may be jealous.
Be happy anyway.
The good you do today, will often be forgotten.
Do good anyway.
Give the best you have, and it will never be enough.
Give your best anyway.
In the final analysis, it is between you and God. It was never between you and them anyway. "
The above poem written on the wall in Mother Teresa's home for children in Calcutta was credited to Mother Teresa. I have italized the last line because it holds the gist of her spirituality.
Being a highly pious person can create a number of problems for the person, including firstly people may be suspicious about your kindness ("He or she has a hidden agenda"), secondly people may exploit your kindness ("You are not kind enough in giving us so little!"), and thirdly your own unconscious can build up a reaction force like a time bomb that is going to explode sooner or later.
The last problem had raised much concerns for Buddhist masters. In Platform Sutra, the Sixth Patriarch raised this issue when he first met the young monk Shun Hui (who later became one of his top disciples). "If I hit you with a stick, do you feel pain or not feel pain....if you feel pain, as a human you are going to bear a grudge against me and will seek revenge one day; and if you do not feel pain, you are not even human, you are just a piece of stone." (my paraphrase)
For Buddhists the solution comes from deep meditation, in particular for Tibetan and Zen Buddhism. Enlightenment is essentially the attainment of inner spiritual experience, which psychologically speaking is a dissolution of our ego without losing control of our personality (losing control will mean insanity). The test of enlightenment is the always the "stick that is going to hit you the next time". For the pious person "He or she has a hidden agenda" is like a stick that hits his or her head.
Mother Teresa once said she had no direct spiritual experience. Hers is another route to spirituality. It is through her strong belief system that is demonstrable or actionable in real life. A belief system that her ego is not responsible to herself nor people around her. Her ego is solely responsible to God, and a most kind God. The criterion of success is demonstrated by one's ability to lead a happy life as a member of Missionaries of Charity for the rest of one's life.
The above poem not only speaks the core of Mother Teresa's spirituality, it also explains what Christianity is all about.