probably the dumbest anti-Christian cartoon I've ever seen

[True Federalist (진정한 연방 주의자)]

I agree, it is fairly dumb, largely because of the strawman it chooses to use.

And if the atheist argument is that John 14:6 doesn't mean that only Christians get into heaven, but rather means Jesus is the one deciding who gets into heaven, then why wouldn't the atheist want to believe in Jesus Christ?

Minor quibble before getting to answering your question, which is already covered in the cartoon.  The other character is only identified as a non-Christian, not an atheist.

That character also indicates he is not interested in the "angry and exclusionary god" that he believes most Christians believe in.  However, he would be interested in the heaven for which Jesus acts as the gatekeeper if Jesus is one who will judge based on the deeds of a person rather than the faith of a person.

[True Federalist (진정한 연방 주의자)]

You're coming at this from the viewpoint of someone who already believes, BushOK, rather than the viewpoint of someone who does not such as the person who made that cartoon.

Indeed, for such a person quoting Romans 3:23 at them is probably reason enough to not be a Christian.  It posits a god who sets a standard of perfection that is impossible for any human to meet, and then threatens everlasting punishment in the afterlife except to those who are willing to pay a bribe in the form of worship to escape his rigged game.  That's not what Christianity really is about, at least in my view, but if it were, I'd reject it too.

[True Federalist (진정한 연방 주의자)]

Yet again, you are presenting Christianity in the form of people having to pay off with worship a vengeful god who is seemingly incapable of the charity he demands humans exhibit.  A person such as the one who made the stupid cartoon that jmfcst posted a link to is not going to be interested in a god who holds humans to a standard that he himself does not meet.