Do Catholics view atheism as a sin?

Do Catholics view atheism as a sin?

The short answer is yes and no. In other words it will depend on the circumstances.

I am going to let St. Thomas Aquinas speak for the Church:

Whether unbelief is a sin?

Unbelief may be taken in two ways: first, by way of pure negation, so that a man be called an unbeliever, merely because he has not the faith. Secondly, unbelief may be taken by way of opposition to the faith; in which sense a man refuses to hear the faith, or despises it, according to Is. 53:1: "Who hath believed our report?" It is this that completes the notion of unbelief, and it is in this sense that unbelief is a sin.

If, however, we take it by way of pure negation, as we find it in those who have heard nothing about the faith, it bears the character, not of sin, but of punishment, because such like ignorance of Divine things is a result of the sin of our first parent. If such like unbelievers are damned, it is on account of other sins, which cannot be taken away without faith, but not on account of their sin of unbelief. Hence Our Lord said (Jn. 15:22)"If I had not come, and spoken to them, they would not have sin"; which Augustine expounds (Tract. lxxxix in Joan.) as "referring to the sin whereby they believed not in Christ."

Atheists that deliberately deny God are guilty of a very serious sin.

Atheism

2123 "Many . . . of our contemporaries either do not at all perceive, or explicitly reject, this intimate and vital bond of man to God. Atheism must therefore be regarded as one of the most serious problems of our time."

2124 The name "atheism" covers many very different phenomena. One common form is the practical materialism which restricts its needs and aspirations to space and time. Atheistic humanism falsely considers man to be "an end to himself, and the sole maker, with supreme control, of his own history." Another form of contemporary atheism looks for the liberation of man through economic and social liberation. "It holds that religion, of its very nature, thwarts such emancipation by raising man's hopes in a future life, thus both deceiving him and discouraging him from working for a better form of life on earth."

2125 Since it rejects or denies the existence of God, atheism is a sin against the virtue of religion.61 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."

2126 Atheism is often based on a false conception of human autonomy, exaggerated to the point of refusing any dependence on God.63 Yet, "to acknowledge God is in no way to oppose the dignity of man, since such dignity is grounded and brought to perfection in God. . . . " “For the Church knows full well that her message is in harmony with the most secret desires of the human heart."

2 Thessalonians 2:12 (NIV) says, "and so that all will be condemned who have not believed the truth but have delighted in wickedness."

I believe an atheist would be one who has not believed the truth. All people have delighted in wickedness.

Yes, atheism is very sinful because there is nothing more hateful of God than to believe He does not exist.

Unbelief is the greatest sin, as St. Thomas Aquinas writes in Summa Theologica II-II q. 10 a. 3 ("Whether unbelief is the greatest of sin?") co.:

Every sin consists formally in aversion from God […] Hence the more a sin severs man from God, the graver it is. Now man is more than ever separated from God by unbelief, because he has not even true knowledge of God: and by false knowledge of God, man does not approach Him, but is severed from Him.