Catholicism states quite clearly that atheism is a sin. Disbelieving God means that you are rebelling against God's commands and therefore sinning.
Atheism, throughout the Bible, is considered "disbelief". The idea is either that you believe in God, or you don't. If you don't believe in the one true God (such as atheism), it's categorized as "disbelief".
Here, we can see the results of not believing in God:
Revelations 21:8 (NIV)Emphasis added
But the cowardly, the unbelieving, the vile, the murderers, the sexually immoral, those who practice magic arts, the idolaters and all liars—they will be consigned to the fiery lake of burning sulfur. This is the second death.”
Disbelief being a sin can also be seen in the Old Testament:
The LORD said to Moses, “How long will these people treat me with contempt? How long will they refuse to believe in me, in spite of all the signs I have performed among them?"
More: Mark 16:16, John 3:18, John 3:36, Romans 14:23b
Catholicism also shows that atheism is a sin:
2125 Since it rejects or denies the existence of God, atheism is a sin against the virtue of religion. The imputability of this offense can be significantly diminished in virtue of the intentions and the circumstances. "Believers can have more than a little to do with the rise of atheism. To the extent that they are careless about their instruction in the faith, or present its teaching falsely, or even fail in their religious, moral, or social life, they must be said to conceal rather than to reveal the true nature of God and of religion."
It uses this bible passage for support of this doctrine:
Romans 1:18 (NIV)
The wrath of God is being revealed from heaven against all the godlessness and wickedness of people, who suppress the truth by their wickedness,
Atheism is one of the gravest sins. It is all throughout the Bible and Catholic church dedicated a section entirely to dealing with the subject (CCC 2123-2126). It is undeniably a sin.