Can atheists go to heaven?

Yes, an atheist may be saved, but not because atheism is a path to heaven. Every person who is saved is saved by Jesus Christ and through His Church, even if that person did not visibly know Christ during earthly life.

What the Church teaches. Jesus is the only Savior: "No one comes to the Father except through me" (Jn 14:6). The Church therefore teaches that all salvation comes from Christ the Head through His Body, the Church (CCC 846). Faith is necessary for salvation, and deliberate rejection of God is a grave matter (CCC 161, 2125). Someone who recognizes the truth of God and knowingly refuses Him cannot presume upon salvation.

What about someone who sincerely does not believe? Personal responsibility depends on knowledge and freedom. Some people have encountered only a distorted picture of God. Others were raised without faith, suffered serious scandal, or reached mistaken conclusions without intending to resist truth. The Church teaches that those who, through no fault of their own, do not know Christ or His Church, but sincerely seek God and strive under grace to do His will as they understand it through conscience, may attain eternal salvation (CCC 847). God can lead such a person toward the faith needed for salvation in ways known to Him alone (CCC 848, 1260).

Atheism can still be culpable. Unbelief is not automatically innocent. A person may avoid God because conversion would require repentance, humility, or a change of life. The Catechism says responsibility for atheism can be significantly diminished by circumstances, but it also warns that believers may contribute to unbelief by neglecting their faith or living badly (CCC 2125). God alone fully knows whether a person's unbelief came from honest ignorance, wounded freedom, indifference, or deliberate resistance.

What Catholics should do. We should neither declare particular atheists damned nor tell them belief does not matter. Christ commands His disciples to preach the Gospel to every nation (Mt 28:19-20). Pray for unbelieving friends, answer their questions honestly, repent of conduct that obscures Christ, and invite them to seek truth. God desires everyone to be saved and to come to knowledge of the truth (1 Tim 2:4), but He does not force anyone to love Him.