If an Atheist does good deeds and serves humanity will he reap the rewards?

If an atheist has lived an otherwise righteous life and live righteously and performed good deeds then certainly it is a credit to them. A person is judged by actions and these good deeds will bear good fruits. This is the law of karma. If someone performs good deeds, it will bear good fruit.

ਕਰਮ ਧਰਤੀ ਸਰੀਰੁ ਕਲਿਜੁਗ ਵਿਚਿ ਜੇਹਾ ਕੋ ਬੀਜੇ ਤੇਹਾ ਕੋ ਖਾਏ ॥
ਗਲਾ ਉਪਰਿ ਤਪਾਵਸੁ ਨ ਹੋਈ ਵਿਸੁ ਖਾਧੀ ਤਤਕਾਲ ਮਰਿ ਜਾਏ ॥
ਭਾਈ ਵੇਖਹੁ ਨਿਆਉ ਸਚੁ ਕਰਤੇ ਕਾ ਜੇਹਾ ਕੋਈ ਕਰੇ ਤੇਹਾ ਕੋਈ ਪਾਏ ॥
“The body is the field of action, in this Dark Age of Kaljug; as you plant, so shall you harvest. Justice is not passed on mere words; if someone eats poison, he dies. O Siblings of Destiny, behold the justice of the True Creator; as people act, so they are rewarded.”
(Ang 308)

Guru Ji also says:

ਸਾਕਤ ਕਰਮ ਪਾਣੀ ਜਿਉ ਮਥੀਐ ਨਿਤ ਪਾਣੀ ਝੋਲ ਝੁਲਾਰੇ ॥
ਮਿਲਿ ਸਤਸੰਗਤਿ ਪਰਮ ਪਦੁ ਪਾਇਆ ਕਢਿ ਮਾਖਨ ਕੇ ਗਟਕਾਰੇ ॥੨॥
“The actions of the faithless cynic are like the churning of water; he churns, constantly churning only water. Joining the Sat Sangat, the True Congregation, the supreme status is obtained; the butter is produced, and eaten with delight. ||2||”
(Ang 982)

The problem with atheism and the problem the atheist person will face however is that internally they will be devoid of the spiritual peace only a belief and experience of God can provide.

ਤੂੰ ਮਨਿ ਹਰਿ ਜੀਉ ਤੂੰ ਮਨਿ ਸੂਖ ॥
ਨਾਮੁ ਬਿਸਾਰਿ ਸਹਹਿ ਜਮ ਦੂਖ ॥੧॥ ਰਹਾਉ ॥
“Believe in the Dear Lord, and you shall find peace of mind. Forgetting the Naam, you shall have to endure the pain of death. ||1||Pause||”
(Ang 226)

True contentment is only possible through spirituality (dharma). Gurbani is very clear that “good action” by itself is of merit, but comparatively little merit when weighed against spirituality:

ਤੀਰਥੁ ਤਪੁ ਦਇਆ ਦਤੁ ਦਾਨੁ ॥ ਜੇ ਕੋ ਪਾਵੈ ਤਿਲ ਕਾ ਮਾਨੁ ॥
“Actions like pilgrimage, self-discipline, mercy and charity are good, but without spirituality, they are of very little weight.”
(Ang 4)

Often what happens is that a person without spirituality who does such deeds is still plagued with restlessness. Sometimes it can lead to ego because in the atheist’s mind, he is the doer. This ego could lead to many other problems for the atheist.

ਸੋਈ ਮਲੀਨੁ ਦੀਨੁ ਹੀਨੁ ਜਿਸੁ ਪ੍ਰਭੁ ਬਿਸਰਾਨਾ ॥
ਕਰਨੈਹਾਰੁ ਨ ਬੂਝਈ ਆਪੁ ਗਨੈ ਬਿਗਾਨਾ ॥੧॥
ਦੂਖੁ ਤਦੇ ਜਦਿ ਵੀਸਰੈ ਸੁਖੁ ਪ੍ਰਭ ਚਿਤਿ ਆਏ ॥
ਸੰਤਨ ਕੈ ਆਨੰਦੁ ਏਹੁ ਨਿਤ ਹਰਿ ਗੁਣ ਗਾਏ ॥੧॥ ਰਹਾਉ ॥
“One who forgets God is filthy, poor and low. The fool does not understand the Creator Lord; instead, he thinks that he himself is the doer. ||1|| Pain comes, when one forgets Him. Peace comes when one remembers God. This is the way the Saints are in bliss – they continually sing the Glorious Praises of the Lord. ||1||Pause||”
(Ang 813)

ਸਾਕਤ ਹਰਿ ਰਸ ਸਾਦੁ ਨ ਜਾਣਿਆ ਤਿਨ ਅੰਤਰਿ ਹਉਮੈ ਕੰਡਾ ਹੇ ॥
“The wicked, the faithless cynics, do not know the Taste of the Lord’s Sublime Essence. The thorn of egotism is embedded deep within them.”
(Ang 13)

There is eternal tension and fear because the atheist feels alone without any help or guardian. And so while s/he may be doing these good deeds, their lives will not be the same as that of a spiritual person.> But once again, to be clear, doing good deeds will not, “make no difference”. It certainly will, but without belief, faith and experience of God through the path of the Guru, they will be depriving themselves of something much greater.

ਮਨ ਏਕੁ ਨ ਚੇਤਸਿ ਮੂੜ ਮਨਾ ॥
ਹਰਿ ਬਿਸਰਤ ਤੇਰੇ ਗੁਣ ਗਲਿਆ ॥੧॥ ਰਹਾਉ ॥
“In your mind, you do not remember the One Lord-you fool! You have forgotten the Lord; your virtues shall wither away. ||1||Pause||”
(Ang 12)