Atheism, in the broadest definition, is the lack of belief in God. It can also be the position that God does not exist. Implicit, or weak Atheism, does not accept God because it sees no evidence of God. Explicit, or strong, Atheism states that the existence of God is impossible. Many Christians believe that atheist are doomed to go to Hell, although some, such as Universalists, Mormons, Liberal Christians, and Jehovah's Witnesses take different stances. Most very religious Muslims also believe that Atheists go to hell.
The Bible warns against Atheism
Psalm 14:1 states, "The fool has said in his heart, “There is no God.” They are corrupt. They have done abominable works. There is none who does good."
There's also a footnote:
Psalm 14:1 The Hebrew words rendered fool in Psalms denote one who is morally deficient.
The Bible says atheists are morally deficient
Therefore, the Bible states that an Atheist is morally deficient. While all people are morally deficient, the Bible specifically condemns Atheism. Of course, the typical atheist believes that this was put in here by primitive men and is not the word of God, so although this argument is sufficient, they believe that it is simply worthless.
Atheists may say the Bible is morally deficient
- Genocide was required.
Deuteronomy 20:14 But the women, and the little ones, and the cattle, and all that is in the city, even all the spoil thereof, shalt thou take unto thyself; and thou shalt eat the spoil of thine enemies, which the LORD thy God hath given thee. 20:15 Thus shalt thou do unto all the cities which are very far off from thee, which are not of the cities of these nations. 20:16 But of the cities of these people, which the LORD thy God doth give thee for an inheritance, thou shalt save alive nothing that breatheth: 
- Christians' Response: The fact is, some of them were heathen, ungodly people. In addition, some were guilty of practicing abominable religious ceremonies, such as throwing their sons and daughters into bonfires for sacrifices to gods like Molech.
- Rape victims were required to marry the rapist.
Deuteronomy 22:28 If a man find a damsel that is a virgin, which is not betrothed, and lay hold on her, and lie with her, and they be found; 22:29 Then the man that lay with her shall give unto the damsel's father fifty shekels of silver, and she shall be his wife; because he hath humbled her, he may not put her away all his days
- Christians' Response: Um, who says this is referring to rape? This just sounds like a prohibition of consensual sex outside of marriage, with the punishment that those who commit fornication must marry one another.
- Arguments for the existence of God
- Agnosticism (neutrality on the question of the existence of God)
- Cosmological argument
- Militant Atheism Exposed
- Reasons that God must exist from a former mislead atheist who was later able to renounce her ways and prove that God is very much real.
- The atheist viewpoint and 50 reasons why they believe God cannot exist (Note that this site selectively quotes. It might be useful to be aware of the context of the bible verses it uses to justify its views.)
- Atheism Wiki