One of many ways atheists contradict themselves and each other is in how they claim man's moral compass should act with regard to God. On the one hand, they respond to the moral argument by saying we should not need God to discourage us from doing evil things (which is actually a straw man). On the other hand, when it comes to the doctrine of Eternal Security (often summarised as "once saved, always saved"), they argue that God does not do enough to discourage us from doing evil things.
The doctrine of Eternal Security states that those who have truly put their faith in Christ are guaranteed to go to Heaven regardless of what they do in this life. This is not the same as being judgement free (e.g. 1 Corinthians 3:9-17), but nevertheless salvation is guaranteed. Hell is off the table. But some atheists argue that this is not good enough. If a Christian is saved no matter what he does, can a Christian rape and kill a number of children and still go to Heaven? How horrific! Yet, as I already alluded to, atheists have the same problem. And yet, they do not have the same solution.
If atheism is true, atheists have the same problem, and arguably a bigger one. See, in atheism, no matter what you do, your ultimate end is the same. Live a "good" life, you die, you forget you even existed. Live a "bad" life, you die, you forget you even existed. Thus, in atheism, there is no motive to alter your life at all. That is, of course, unless you consider the smaller picture. But with the bigger picture in mind, there is no motive to live a "good" life. Furthermore, atheists have a bigger problem, because if atheism is true, there is no such thing as "good" or "evil". Rather, moral actions are judged by another equal mind. What one man thinks is "good", another may think is "evil", and neither of them have solid ground (key word: solid) to prove the other wrong.
But in Christianity, here's the solution: God. See, if the Bible is true, good is good even when it lacks reward, and evil is evil even when it lacks punishment. We should not need motivation. Motivation such as reward is a bonus, and motivation such as punishment is surplus to requirement. Reward is given out of the generosity of God's own heart, and punishment exists only for when good is not respected.
Furthermore, it just makes sense. If you know someone will forgive you for doing them wrong, is that motivation to continue doing them wrong, or is it perhaps motive to love them all the more? If you take the latter option, congratulations, you have a Christian mindset. We love God because He first loved us (1 John 4:19). If you choose the former option, what does that say about you? Let's keep this article "family friendly" and just say that it makes you a bit of a jerk.
You're also extremely fickle, and that isn't the kind of faith that saves in the first place. God doesn't take sinners who are scared of punishment, remove their fear of punishment and let them run riot. God takes bad people and sets them free before building them up to be good. The kind of faith that says "I'm not sorry for doing evil, I'm sorry I got caught doing evil" is the exact opposite of the Christian faith. Sure, theoretically, a Christian could commit any number of atrocities and still go to Heaven, but his commitment of those atrocities suggests that he was never bound for Heaven in the first place.
To quote Martin Luther, "salvation is by faith alone, but faith will never be alone". To quote James, the brother of Jesus, "Show me your faith without your works, and I will show you my faith by my works. You believe that there is one God. You do well. Even the demons believe—and tremble! But do you want to know, O foolish man, that faith without works is dead?" (James 2:18b-20).
Our actions reveal our faith, because our faith motivates our actions. As Christians, we are still in the flesh. We're still sinners. We still have human desires. Wealth, food, sex, the admiration of others, self-preservation, all these things and more are motivating factors that unfortunately mean as long as we are in this body, we will still sin. But if our faith is in God, truly in God, we will also be at war with sin in spite of these things. I don't need money as much as I need God. I don't need food as much as I need God. I don't need sex at all, but I need God. I don't need to be admired at all, but I need God. And at the end of the day, I need God so much that if push comes to shove, I ought to die for Him.
And so you see that while atheism necessarily leads to the rejection of religious beliefs, such as moral values and a meaning for life, Christianity necessarily leads towards them. We were created by a Holy God for the purpose of bearing His image. We are supposed to walk like Him, talk like Him and even think like Him. There's only one true thing about the atheist argument: Neither fear of punishment, nor hope for reward, should even enter our minds when we consider the question "what should I do?" The fact that it very often does is just an unfortunate result of our own sinful nature.
And that really is the point. Atheist or Christian, we're all sinners. Even atheists, though they may claim they are "good without God", are, in fact, sinners. They may be good in their own eyes, because they have set up such loose standards that it would be virtually impossible for them not to be "good" in their own eyes, but the truth is, every single one of us has violated God's laws. And that does include me. I'm not going to pretend I'm going to Heaven because I deserve it. I didn't work my way there. I didn't pay my way in. When I go to Heaven, I will go as a filthy rag who deserves nothing less than the fullness of God's wrath. But in His infinite mercy, He looked down on me and had compassion, instead sending His beloved Son to receive that wrath on my behalf. Through faith in Him, I then receive the reward I would have if I had lived as He did. That is how I'm getting to Heaven. And my atheists friends, today I offer you the same invitation I was offered. As a fellow beggar, this is where I found bread: In the loving hands of the Lord Jesus Christ, who is not willing that you should perish, but would rather you turn from your sin and live. Your motivation? God.