Why do some Christians believe if an Atheist does something good it cannot be considered a good deed?

This confusion seems to prevail on both sides. There is a particular classical Christian argument against atheism that some Christians tend to warp out of recognition, and some atheist become so offended by that they warp out of sanity.

The issue is not weather an atheist cannot do anything that would be considered good. The Christian charge is not that he is unable to obey civil law, show compassion, be charitable or the like. The issue is philosophical... it is that the atheist has no rational, consistent basis for a standard of "GOOD" by which any action (call it 'good' or 'bad') can be judged in any way other than in a completely arbitrary manner.

The argument doesn't end there...

The Christian will then assert that atheist do know the basics of right conduct, and that they cannot help but know them... For God's collective grace has manifested it unto them. Furthermore, the atheist knows that he is not in compliance with - what C. S. Lewis called - this Law of Human Nature. The Christian knows that the atheist is not as bad as she could be, or as she someday will be apart from conversion. But the whole point is that the atheist's own inconsistencies in this area are the very evidence of God that the atheist claims does not exist... The evidence of God is so close and so entwined within the atheist and the world, that the atheist knows in her heart that God exists, and she is suppressing that knowledge all the time.

any one who does any thing nice it is not considered by god as good as god defines good

it does not qualify a person to go to heaven

god says no one is righteous as god determines righteousness

righteous is being good, holy, pure

righteousness is doing good or holy or pure things

while humans helping some one is considered good to god its superficial

doing a good deed is not a reflect of a persons heart

humans think it is but god says its not

man's heart seeks to do evil

a evil man can do good deeds

that does not make him a good person just because he does a good deed

some of the most notorious killers have done good deeds

the BTK killer did work for a church and camp.

john wayne gacy entertained children

http://www.time.com/time/2007/crimes/13.html

But John Wayne Gacy was also an upstanding citizen: he helped out the neighbors, he was the chaplain of the Jaycees, he dressed up as roly-poly Pogo the Clown to entertain children

just because any person does something good does not mean he is pure of heart

Romans 2

13 For it is not those who hear the law who are righteous in God's sight, but it is those who obey the law who will be declared righteous.

14 (Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law,

15 since they show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts now accusing, now even defending them.)

Good deeds done for good reasons by non-Christians have the same value as those done by Christians.

whoever told you that is a lunatic

I don't think that way, and I don't know one other Christian with that view

some people have a morbid hatred of Christians and God, and want you to follow

my Dad was an atheist, and he was the kindest, most wonderful father. . wouldn't have swapped him for anyone.

It's what churches teach. Nothing is really good if it isn't done to glorify God, or non-believers only do good for selfish reasons, or it only appears to be good and if you looked a little further you'd find an ulterior motive. They teach that only actions done for the glory of God can be considered truly good.

Well, for Calvinists no deed is actually good. This is known as total depravity: humanity as such has no redeeming features. According to them only God is good, and only God can save us.

Because they stroll the streets with a "Holier then thou" attitude and believe that they're special because they worship a 2000 year old cannibal Jewish zombie.

I don't think any atheist really cares what some christians think, I certainly don't.

It's illogical for you to leap to the conclusion because ONE Christian said this to you.

Why do you do that?

Without loving and obeying your parents, whatever good you do for others shall not be counted as good by your parents. First you love them and then help others. This is the case with atheists.