It is not uncommon for us to say or hear someone declare that I am a good person. While the intent behind the statement may be good, here are some reasons why it falls short of being a good thing to say:
1. How Are You Sure You Are Good?
Saying “I am a good person” begs the following question: how do you know and what criteria did you use? Without a clear definition and criteria for measuring “good”, the mere declaration that I am a good person is not meaningful, useful or reliable information.
2. Only God Can Decide if You Are Good
For the declaration to carry any weight, there has to be a generally-accepted, time-tested and true measure of a good person. That measure must be generated by a purely good source. Essentially that measure is God and His Word since God alone is totally good – yesterday, today and forever (Mk 10:18).
3. You Cannot be the Judge of Your Own Actions
Declaring that you are a good person makes you the judge of your own matter. This increases the chances of bias and makes the declaration unlikely to be accurate. The declaration requires (not you but) a good judge (God) who uses good criteria.
4. The Church (Not Individuals) Confirm Some Actions as Good or Bad
God has delegated some of this authority to judge actions as good or bad to the Catholic Church. This is known as the teaching authority or Magisterium of the Church. In addition, He gave the Church the authority to forgive moral errors (or sins) : John 20:23. This is why going to the sacrament of Confession is truly a privilege and can help you to be a really good person in the sight of God.
Side Note: The Church, through its process of declaring Saints, a.k.a Canonization, may find that a person who has died, led an exemplary life of heroic virtue i.e. he or she is found to have been a good person. This is a thorough process involving extensive research, multiple witnesses and other data. It is not the same as an individual making a self-declaration that he or she is a good person.
5. Apply the Right Criteria
The correct criteria to decide what is good have been provided and sanctioned by God, who is the source and definition of goodness.
To test whether an action you take is good, do not measure it by how other people will view you or by what is popular, trendy or generally accepted by people around you. Popular thinking has not always been good. Take for instance institutional slavery and the extermination of Jewish people in Europe in the 20th century. These were based on popular belief and were morally wrong.
Using the above criteria, most of us will find that we do well in some areas and fail in others. Take your failures to Confession and begin again with a resolution to choose the good.