Atheism

The error of those who deny the existence of God. Practical atheism consists in living as if God were not. Speculative or theoretical atheism consists in the assertion that there is no God. (Definition from A Catholic Dictionary, 1951)
 

References in Scripture:

• "For God sent not his Son into the world, to judge the world, but that the world may be saved by him. He that believeth in him is not judged. But he that doth not believe, is already judged: because he believeth not in the name of the only begotten Son of God." John 3:17-18
• "The fool hath said in his heart: There is no God" Psalms 13:1
• "But all men are vain, in whom there is not the knowledge of God: and who by these good things that are seen, could not understand him that is, neither by attending to the works have acknowledged who was the workman....
  But yet as to these they are less to be blamed. For they perhaps err, seeking God, and desirous to find him. For being conversant among his works, they search: and they are persuaded that the things are good which are seen. But then again they are not to be pardoned. For if they were able to know so much as to make a judgment of the world: how did they not more easily find out the Lord thereof?" Wisdom 13:1,6-9
• "But it is impossible to escape thy hand. For the wicked that denied to know thee, were scourged by the strength of thy arm, being persecuted by strange waters, and hail, and rain, and consumed by fire" Wisdom 16:15-16
• And to you who are troubled, rest with us when the Lord Jesus shall be revealed from heaven, with the angels of his power: In a flame of fire, giving vengeance to them who know not God, and who obey not the gospel of our Lord Jesus Christ. Who shall suffer eternal punishment in destruction, from the face of the Lord, and from the glory of his power" 2 Thes 1:7-9
   

Church Teaching:

• "During Our Pontificate We too have frequently and with urgent insistence denounced the current trend to atheism which is alarmingly on the increase" ENCYCLICAL OF POPE PIUS XI ON ATHEISTIC COMMUNISM MARCH 19, 1937
• "That which seems to Us not only the greatest evil but the root of all evil is this: often the lie is substituted for the truth, and is then used as an instrument of dispute. On the part of not a few religion is passed by as a thing of no importance, and elsewhere absolutely prohibited in family and social life as a remnant of ancient superstitions; public and private atheism is exalted in such a way that God and His law are being abolished, and morals no longer have any foundation. The Press also too often vulgarly reviles religious feeling, while it does not hesitate to spread the most shameful obscenities, agitating and with incalculable harm leading into vice tender childhood and betrayed youth" Encyclical On a Program for Combating Atheistic Propagandas throughout the World, By Pope Pius XII, 1950
• "Certainly such attempts can nowise be approved by Catholics, founded as they are on that false opinion which considers all religions to be more or less good and praiseworthy, since they all in different ways manifest and signify that sense which is inborn in us all, and by which we are led to God and to the obedient acknowledgment of His rule. Not only are those who hold this opinion in error and deceived, but also in distorting the idea of true religion they reject it, and little by little, turn aside to naturalism and atheism, as it is called; from which it clearly follows that one who supports those who hold these theories and attempt to realize them, is altogether abandoning the divinely revealed religion" ON RELIGIOUS UNITY
  ENCYCLICAL OF POPE PIUS XI JANUARY 6, 1928
• "These reasons suffice to show superabundantly by how many roads Modernism leads to atheism and to the annihilation of all religion. The error of Protestantism made the first step on this path; that of Modernism makes the second; atheism makes the next" ON THE DOCTRINE OF THE MODERNISTS
  ENCYCLICAL OF POPE PIUS X, SEPTEMBER 8, 1907
• "First of all, a faith proud, alert, fearless, firm and alive to truth, to the triumph of Catholic doctrine. The intellectual and political forces more or less impregnated with atheism are set to destroy Christian civilization"
  Allocution of Pope Pius XII to the Congress of the International Union of Catholic Women's Leagues, Rome, Italy, September 11, 1947
• "But, in a word, atheism is so monstrous an error that it could never, be it said to the honor of humanity, annihilate in it the consciousness of God's claims and substitute them with idolatry of the State." ON THE CHURCH AND STATE IN FRANCE, Encyclical of Pope Leo XIII promulgated on February 16, 1892
  
   

Summary

As Catholics, we shouldn't need to comment on atheism; it is the absence of all religion and we can see Scripture and Church Teaching are directly opposed to this.

A widespread error that does concern Catholics today is religious indifference; which allows religions of any kind, and this inevitably includes, or at least leads to, atheism, as Church teaching above clearly states. Anyone who condones non-Catholic religions (or no religion at all) cannot call themselves Catholic!