There is nothing more ironic than an atheist trying to argue morality. Atheism, as a worldview, is necessarily amoral, and yet atheists are by far the most vocal minority when it comes to their moral views.
This is especially the case when those atheists happen to be Leftists. Oppose abortion, you're a misogynist. Criticise Islam (because apparently there's only one religion atheists really hate), you're Islamophobic. Criticise gay "marriage", you're homophobic. Believe the basics of biology, you're transphobic. Support immigration controls, you're a racist. The list goes on. Some atheists even call it child abuse to teach children about Christianity.
But here's the question: If we're evil, what god should we repent to? If there is no God, morality is relative. There is no such thing as good or evil, there is no sin, there is no morality, we live in a blind and pitilessly indifferent universe. Only if there is a God can there be moral absolutes, but if atheism is true, morality is entirely subjective. Basically, even if Christians were misogynistic, Islamophobic, homophobic, transphobic, science-denying, racist, hate mongering, child abusers, (which of course we are not), that wouldn't be morally wrong, it would simply be disliked.
This is one of my favorite arguments to use against atheism, because they simply cannot disagree with it without proving it. Every time an atheist tries to come up with an argument for why moral laws can exist in an atheistic worldview, they inevitably end up appealing to a subjective source, whether it be to an individual human, or to a group of humans. And in true atheist fashion, whenever you point to a similar source that would result in a moral view they disagree with, they hypocritically disagree with that source, because in truth, it is not their true source.
For example, the most common attempt to justify moral absolutes I come across is the law. But of course, different jurisdictions have different laws, the same jurisdictions frequently change the law, and those jurisdictions all have laws that no atheist will agree with every time.
In Pakistan, atheism is effectively illegal. There is no specific law that says "you cannot be an atheist", but blasphemy (which would include the denial of Allah's existence) is punishable by death. Would any atheist agree that this is morally acceptable? If so, then either morality really is relative, or morality does not depend on the government.
In the UK, gay "marriage" is legal now, but it wasn't until 2014. If the government is the standard, gay "marriage" was evil when it was illegal, but is now not evil. This would mean morality is relative. And again, there are other countries where it is still illegal. If it is evil there, but not here, then once again morality is relative.
In America, the second amendment effectively makes gun control illegal. That doesn't mean there are no restrictions on gun ownership, but it does mean that most of the restrictions that do exist are constitutionally illegitimate, and that a complete ban on firearms, like the asinine laws we see here in England, are theoretically impossible to be made law in America. Once again we have the relativity problem if the government is the standard. But many atheists argue that the law should be changed. But if it should be changed, that means the government cannot be the source of morality, because there is something higher than it to which it must submit. This puts atheists back at square one: What is that standard?
I could go through all of the different attempts atheists use to justify the existence of morality, but suffice to say they're all just as weak. Atheism is a morally relativistic philosophy, and thus they have no basis to call anything evil, including actual evil. An atheist like Richard Dawkins cannot even call Joseph Stalin (who was also an atheist) evil. And so how can they ever call a Christian evil? By what moral standard do atheists judge, and to what moral source do atheists believe we are accountable? There are none. Atheism is amoral.
Needless to say, Christianity isn't. When a Christian says something is wrong, the answer to the question "what God should I repent to?" is the God of the Bible. His moral laws are absolute, and we will be held accountable to Him one day. Tragically, we have all done something that offends Him, and so repentance is essential.
But is it effective? After all, when you break the law, you can't exactly walk into the courtroom expecting "I'm sorry" to earn you your freedom. Even if you are genuinely sorry for committing the crime, a just judge will ensure you serve the appropriate sentence. And God is the just judge. But the good news for us is that God deliberately left a loophole available to us. Repentance does work, not because God is unjust, but because an innocent person took our punishment on our behalf. Jesus, the Son of God, came to earth as a man, lived a perfect life, and died on the cross. In order to receive a full reprieve, all we need to do is confess Jesus as Lord and believe in our hearts that God raised Him from the dead. You do that, Jesus presents you blameless before God. That is the God to whom you should repent.