The Catechism of the Catholic Church says
2125 Since it rejects or denies the existence of God, atheism is a sin against the virtue of religion. 61
61: Cf Rom 1:18
Sin can be expiated, of course, so those who are currently atheists may reach heaven. But heaven will [does] contain no atheists.
In saying “But do good: we will meet one another there,” the Pope means that people will meet each other in doing good, not necessarily in heaven. That is, there is common ground in altruism.
In saying “The Lord has redeemed all of us, all of us, with the Blood of Christ: all of us, not just Catholics. Everyone!” the Pope is stating a truth, expressed also in the Catechism (my emphasis):
851 Missionary motivation. It is from God's love for all men that the Church in every age receives both the obligation and the vigor of her missionary dynamism, "for the love of Christ urges us on."343 Indeed, God "desires all men to be saved and to come to the knowledge of the truth";344 that is, God wills the salvation of everyone through the knowledge of the truth. Salvation is found in the truth. Those who obey the prompting of the Spirit of truth are already on the way of salvation. But the Church, to whom this truth has been entrusted, must go out to meet their desire, so as to bring them the truth. Because she believes in God's universal plan of salvation, the Church must be missionary.
343: 2 Cor 5:14; cf. AA 6; RMiss 11.
344: 1 Tim 2:4.
But in order for atheists to be saved, they need to come to the knowledge of the truth. The Church needs to impart that knowledge (because it is the Church to whom the truth has been entrusted). Once they have knowledge of the truth, they will no longer be atheists.
The starting-point is the common ground of doing good together, and the Church may say that the innate knowledge that doing good is good is God-given; and that good acts are prompted by the Spirit. “Those who obey the prompting of the Spirit of truth are already on the way of salvation.”
Rom 1:18 — “The wrath of God is revealed from heaven against all ungodliness and wickedness of men who by their wickedness suppress the truth.”
2 Cor 5:14 — “For the love of Christ controls us, because we are convinced that one has died for all; therefore all have died.”
1 Tim 2:4 — “[God our Savior,] who desires all men to be saved and to come to the knowledge of the truth.”