Questions you can ask in return:

Who defines goodness?
Many people have different ideas on what constitutes goodness.  Some like country music, some like rap music.  Some like chocolate, some like vanilla.  Most consider betrayal and murder as morally bad but some tribes  in history have considered betrayal and murder as some of society's highest ideals.  Who is right?  Is there a standard?

Doesn't it make sense that God, the Creator of all things should determine the standard for what is good?
If God is the Creator, doesn't it make sense that His rules apply, that what He says goes?

Have you been good based on God's standards and what He considers good?
He considers dying to self and loving God first and people second as the most important things in life.  Have you always done that?  Have you served yourself or others?  Have you broken his rules (the 10 commandments and others)?  Have you ever lied, cheated, stolen, lusted, loved money, been rude and disrespectful to your parents?  Have you failed to give up what you want so that others can be blessed?  If so, you are not good based on God's standards.

You might be good compared to other people.  Is that what you meant by "I am a good person?"
Some people are genuinely good when compared to others.  Mother Teresa and Gandhi definitely seem better than Adolph Hitler or Idi Amin on the goodness scale.  However, if Hitler is a "1" on the goodness scale and Mother Teresa is a "100", God is 1x10^9999 on the same scale.  (He is actually off the scale.)  A sheep looks brilliantly white against a field of green grass, but the same sheep looks filthy compared to a field of freshly fallen snow.  It is important that we weigh our goodness against the purity and righteousness of God and not against others.  To compare ourselves to others in terms of goodness is unwise.

Do you believe people are inherently selfish?
From the time a small child is born it knows how to live for itself.  If you take a five year old and put him in a football field full of toys but tell it not to play with one particular toy and then walk away, chances are the child will play with that one toy.  As we grow up, we continue to live for ourselves, doing our own thing, living our way, setting our own pace in life.  We tend to live for self.  Every "sin" we can think of usually begins with selfishness, whether lying, stealing, murdering, or lusting, our sin is rooted in selfishness.

Interestingly enough, the main tenant of Satanism isn't baby sacrifice, reciting the Lord's prayer backward, or anything of the sort.  It is simply, "Do what thou wilt", which means, "Do your own thing."  Have you always done your own thing?  If you have, God does not consider you good.

God's Kingdom (and the Kingdom of Heaven) is based around two central tenants: love God (outside of yourself) and love others (outside of yourself).  Based on these two tenants, have you actually been good?

Isn't it kind of arrogant to say we are "good"?
Jesus says that the humble will be exalted and the proud will be brought low.  Isn't it pride (and if we are honest - a lie) to say we have been "good"?  If that is the case, isn't our own pride something specific we can look at to see that we are not actually good anyway?  If there was one sin that Jesus hated it was self righteous pride.  Isn't it possible that just thinking that in and of ourselves we are "good" that we actually become bad?

Jesus tells the story of a Pharisee (religious leader) and a Publican (a bad man, a sinner).  When the two go to the temple, the Pharisee brags on his own goodness.  The Publican cannot even look up to Heaven but instead bows down and confesses that he is a sinner.  Jesus says that it is the Publican, not the Pharisee who goes home right with God.

Do you know how to become truly good?
There is only one major religion in the world that offers what is called the "Atonement" which basically means, cleansing and healing from the past.  Even if, from here on out each of us could be truly "good" (which most cannot or will not) it doesn't change the fact that our past contains sin.  Only Jesus offers cleansing from the past, a new, clean slate, and a new life from the inside out.  Only Jesus allows a person to be "born again" and start over.  It is by receiving Him that we can be truly clean.

Related Bible verses:

Romans 3:9-18                   Romans 7:18                 Galatians 3:22

2 Corinthians 10:12           James 2:10                    

Related quotes:

"Nothing makes one so vain as being told that one is a sinner."
                                                                         -Oscar Wilde

"Let us not be guilty of mistaking people who do good things for good people.  There is a difference. . . . Hitler was Time Magazine's "Man of the Year in 1938."                       From God's Greatest Passion (p. 132)
                                                   Order it HERE.

Related media:

Dog Poo and the Gospel: a study in Goodness 
What is Goodness?  What does God consider good?  Am I good?  Can a little bit of bad keep me from being good? Watch this video from H.L. Hussmann and find out.

Hell's Best Kept Secret
Well known sermon from Ray Comfort in audio format. This covers the idea of using the Ten Commandments in a witnessing situation to effectively bring people to a place of repentance before God.

Related Websites or Articles:

ARE YOU A GOOD PERSON?
Take this test to find out.

THEN: Life now. Life Then. Life after death.
An article from everystudent.com addressing the question: How good do you have to be?

Related Books:

God's Greatest Passion H.L. Hussmann