What do you think is the atheists strongest argument against Christianity?

"I am a Christian"

Ok.

"I have my opinion as to what the strongest argument might be that would hinder an atheist from becoming a believer"

My ability to clearly separate fantasy and reality.

A good education helps with that.

Stories about gods are not plausible.

The idea that there might be an invisible magic being in the sky is just offending to an educated mind.

There are many very different stories about many very different gods. There is nothing that suggests that any of them might be a true story. All civilizations had their own religion with their own gods. Later they all turned out to be nothing but stories. No exception. And there is no reason to assume that todays religions are any different in that regard.

"but I was wanting some input from others."

You are welcome.

"Please keep this civil and please don't give a flood of different arguments."

Why no flood?

Religion is wrong on so many levels.

Australia , a place time forgot and so did the gods , they left it the the mother earth and gave us wattle

And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil. 

For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved. 

So much for you being a Christian !

There aren't any. The reason why atheists become atheists is not based on reason or logic, despite that common claim. The main reason why they argue against Christianity is that it goes against their worldview.

Christianity points to a loving God, one with so much love for humanity that he arranged to sacrifice his most beloved son in order for us to reconcile ourselves to him. Atheism is based on believing that any god is a creation of humans, and any story about God is a fiction. But all fictional god stories describe someone with human flaws - all except one.

The true God's love is so encompassing, so self-sacrificing, that it is greater than anything that a human would be willing to imagine, show, or even have. And Jesus' death and resurrection are not just proven by some ancient scroll, but by the testimony of the sacrifice of the first century Christians who would rather die than deny him. This truth cannot be denied, because it is even attested to by people who hated Christianity - the Romans and the Jews of the first century.

The real and true God would not let that testimony die with his Son. The single most printed, distributed and sold book in the entire world history is the one that tells the account of his human ancestry, life, death and resurrection, and also the accounts and words of his followers who would not keep silent, even at the risk of their lives.

Atheists who have been willing to move their world view aside for just a moment, long enough to grasp the uniqueness and gravity of this testimony, have been able to see and sense the One who is invisible, and develop faith in this true God, and His plan to reconcile thos of humankind who are willing to be drawn to Him. through His Son and our Redeemer, Jesus Christ.

I don't think they have a argument.. their just waiting to see something go haywire . like it already hasn't in the catholic religion and say ... oh we knew all along

The strongest involves many of the negative behavioral practices (killing those who are not one of "us", persecutions, etc) and many of the supernatural claims (many of which could be understood metaphorically based on the writings of the era).

It should be, "Because there are no gods, there's no need to argue against Christianity any more than there's a need to argue against all other religions. We can dismiss the lot of them, without argument, because there are no gods."

Yet, somehow, that never is their line, because they just want to argue, and predominantly against Christianity which obviously is the greatest, felt danger to atheism. Now, why would that be, if - as atheism claims - there simply are no gods? Methinks they protest too much.

The NT, and all the lies in it.  Those so-called NT prophecies came by way of  writers having plagiarized the OT. 

I choose to base my beliefs in empirical evidence. So far, I haven't seen empirical evidence for the existence of God.

This is tricky because atheists know different things where not all atheists are familiar with Christianity and it's history and simply see the premise as flawed, meaning no good reasons to support even the remote possibility of Gods, making all God based religions false concerning any Gods. I am however very familiar with the Christian religion and it's history where the strongest argument against Christianity is the bible itself. The NT writers, based on how they wrote and the sources they clearly used to reverse engineer the character of Jesus is a dead giveaway that Christianity is indeed made up with an agenda in mind. In a nutshell, the NT writers often take references to Israel and its people and substitute their character of Jesus. One example is the suffering servant of Isaiah 52/53 that christians will tell you is a prophecy for the future Jesus. However if you read further back leading up to this, Isaiah several times tells you that the servant is Israel and is explaining why the Hebrews had to suffer the captivity by the Babylonians, where Isaiah even tells us using the "past tense" that the suffering had already occurred. There are many, many of these kinds of examples of the NT writers misrepresenting the OT for the sake of their character Jesus and the story/message they wished to convey. Learning the history also helps to understand just how menacing Christians were with most external sources referring to Christians and the supposed Jesus in an unfavorable light.

Simple.

The absolute and total lack of any indication of a god ever existing