Why do Catholics believe in purgatory?

Catholics believe in purgatory because Scripture and Tradition affirm that nothing impure enters heaven (Rev 21:27), yet most souls die in God's friendship but still imperfectly purified. Purgatory is the final cleansing God mercifully provides so the soul can fully enter his presence. The Church has defined this as dogma.

The Scriptural Foundation

Revelation 21:27 states plainly that nothing defiled shall enter the heavenly Jerusalem. This demands a purification for any soul that dies in grace but still carries the residue of sin's disorder.