Many theists believe it is clear-cut. Humans can only have opinions about morality, and no one’s opinion is any more valid than anyone else’s. This leads them to the conclusion that an objective source of morality must stand apart from, and above, humans. That source, they say, is God. Since atheists, reject God, atheists can have no basis for morality.
This is really two separate arguments: (1) that God is the source of objective morality and humans can learn morality from God and (2) that humans on their own have no way to know what is moral and what is not.
Can atheists be moral? - Atheist Alliance International
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Morality is a human concept and not a Godly standard.
As Christians we seek to act in accordance with the perfect will of God. To act contrary to the perfect will of God, no matter the reason, is sin.
Can atheists accidentally act in a manner which is consistent with the perfect will of God? Absolutely.
Can they always act in a manner which is consistent with the perfect will of God. Absolutely not...but take heart atheists, neither can Christians.
For all have sinned and fall short of the glory of God - Romans 3:23.
God's standard is perfection and none of us can be perfect (save God).
Sadly, I don't believe that even using the human standard of morality, there isn't a human that can act morally all the time.
To bridge the gap in language between atheist and Christian let's exchange the terms morality and sin for selflessness.
To do the perfect will of God one must continually submit their own selfish will to the perfect will of God and place the needs of others above their own. Any honest Christian will admit that they struggle to put the needs of others ahead of their own.
Human morality seeks to produce ethical and "good behavior" by restricting one man's behavior so s moral person's actions don't adversely affect others (sound familiar). Most honest atheists will admit they struggle to do so.
There are some Buddhist Monks who might argue they have achieved this standard, but there is one additional problem with that argument.
Under the Godly standard, it not sufficient to act correctly, because thinking of acting sinfully is the same as actually taking the action.
If you can show me the other person who has ALWAYS thought and acted selflessly (there was one), I'd really like to meet that person.