The afterlife is a huge area of objection for atheists. A number of questions are often asked. Why doesn't God just forgive everyone and let us all go to Heaven? Why would God allow (insert heinous sinner here) just because they repented and believed? Why won't God allow "good" people to Heaven just because they don't believe in Him? Questions like that often come into theological discussions, but at the end of the day, Heaven, Earth and everything in them are God's, and God's alone. For that reason, He has every right to dictate what goes on in all these realms. If He wants to set a certain rule, it's His right to make it. If He wants to send a certain sinner to Hell, He has that right. If He wants to forgive a certain sin, so be it.
The question to ask of the atheist is this: knowing that you can be forgiven for your sins and inherit the kingdom of Heaven, why wouldn't you take that option? Instead of complaining that He won't let you in if you don't believe, try believing and get yourself let in. I will never understand how atheists can spend so long objecting to the existence of Hell, yet follow it up with "therefore, I insist on going there, and taking as many people as I can down with me!"