Atheist and Christian why wearing revealing clothes is a sin?

There's no such thing as sin.  That's a fake word.  And, wearing revealing clothes is fine, as long as you want men to think of you as a sex toy, rather than a person...

And so you seem to feel what you believe is of some significance it seems.

it is a sin to intentionally cause a brother or sister to stumble.

Therefore if YOU wear something that YOU KNOW will cause others to LUST, then you absolutely commit sin by doing so. And not only that, you commit that sin each and ever time a person LUSTS because of what YOU chose to wear.

If YOU wear something that YOU don't think will cause anyone to Lust, and yet they do lust, that is NOT a sin counted unto YOU, because YOU were not trying to cause others to LUST, or thought it would cause others to LUST.

Modesty is not something you can tell OTHERS how to be. Every person should judge within their own selves what they consider to be modest, and let not others judge others by what they themselves think is modesty and what is not.

For example a woman can dress is such a way, that she genuinely believes will not cause anyone to stumble in lust, but then a person who does LUST after her, might judge her to be dressing immodestly, which she did not do.

But woe to any person who dresses in such a way, that they KNOW will, or may, cause some to LUST. They have altogether NOT dressed in a Modest way, if they thought it may cause others to stumble.

They encorage temptation and lust. And thats the goal of the wearer...to gain attention. They've nothing else to offer. 

tempt·ress

/ˈtem(p)trəs/

noun

a woman who tempts someone to do something, typically a sexually attractive woman who sets out to allure or seduce someone.

It is always religion that makes people cover up. In muslim countries women are covered from head to toe. 

A long time ago in Hawaii, people wore grass skirts. men and women, with not much else because it is a hot place and they were comfortable. Then the white man conquered Hawaii and brought in Christianity. They made women dress in very conservative clothes because of the Bible and religious teaching. 

I'm not a Christian, but the Bible says...

Timothy 2:9 Likewise also that women should adorn themselves in respectable apparel, with modesty and self-control, not with braided hair and gold or pearls or costly attire,