“All who die in God’s grace and friendship, but still imperfectly purified, are indeed assured of their eternal salvation; but after death they undergo purification, so as to achieve the holiness necessary to enter the joy of heaven.”
Catechism of the Catholic Church, 1030

For Reflection:
How does Purgatory reveal to us the mercy of God? Why is purification and true holiness necessary to enter the joy of heaven?