The Pope made an off the cuff remark, that though true, is very easy to misinterpret. Much like with the Bible, a comprehensive understanding of the subject is essential to grasp the truth.
From the Catechism of the Catholic Church: 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."
In other words, as I have believed for a long time, we as Christians should have special consideration for atheists, because so many have turned from the Church because of the bad examples of people calling themselves Christian.
Is it possible for atheists to gain heaven? Yes. But the "intentions and circumstances" of the atheist bear very close examination. For example, I have read where some Holocaust survivors ceased to believe in God. I have no doubt that a human being can be so wounded by their circumstances in life that they can look out into the world and see no justice, and thus see no God.
Please note that Pope Francis did NOT tie gaining Heaven with good works. He explicitly said conscience is the wellspring of worthiness of Heaven. Attendance to one's conscience is an expression of faith, an unspoken and perhaps unacknowledged belief in the power of God, because God is the source of conscience.
